Position rack in lower third of oven and preheat to 300 degrees. Pour the flour and 7 tablespoons of sugar in that order into a triple sifter or sieve. Sift onto a sheet of waxed paper, and set aside. (Sifting the two together disperses and separates the particles of flour so that when dry ingredients are added later to the batter, folding in the flour is easier and more efficient, requiring less folding to incorporate. Measure the additional 7 tablespoons and 1 tablespoon of granulated sugar; set aside.
Separate the eggs, placing the whites in the bowl of a heavyduty mixer and yolks in a 1 1/2 quart bowl. (A deep bowl this size makes it more efficient for whipping the yolks thoroughly since it minimizes the surface area.) With an electric hand mixer, whip the yolks on high speed (#10) for 3 to 5 minutes. Add the vanilla, and continue whipping for 15 seconds to incorporate; the mixture should appear thicker, pale yellow, and increased in volume. Test the consistency by lifting some of the mixture with the beaters. If it flows back into the bowl in ribbons that slowly dissolve on the surface, proceed to whipping the whites. But if the ribbons sink into the surface immediately, continue whipping until the yolks have the desired consistency.
Attach the bowl of whites to the mixer, and with the whisk attachment, whip on mediumlow speed (#3) for 30 to 45 seconds to break them up. When small bubbles appear and the surface is frothy, stop the machine, and sprinkle the cream of tartar and the 1 tablespoon granulated sugar in the center. Resume whipping, increasing speed to medium (#5), for 4 to 5 minutes, or until the whites appear glossy and stiff but not dry or granular.
Detach the whisk and bowl, tapping the whisk against the side of the bowl with enough force to free the excess. Pour the yolks onto the whites (notice they float on the surface). Using just a few strokes, fold the two together with a rubber spatula. Don't be concerned if some of the yolks remain visible. Sprinkle half of 7 tablespoons sugar over the surface, and fold to incorporate. Sprinkle with the remaining sugar, folding again to incorporate.
With the aid of a metal spatula, scoop a third of the flour, and sprinkle it over the egg mixture; with a rubber spatula, fold to incorporate. Repeat two more times, folding just until incorporated after each addition.
Gently pour batter into an ungreased 9-inch springform pan with the aid of a rubber spatula and smooth the surface level. Bake for 55 to 60 minutes, or until the top is golden. (The batter hardly rises during the first 20 to 30 minutes of baking. The low oven temperature allows the batter to absorb heat slowly, so rising is gradual. Toward the end of baking the cake may mound high above the pan's rim. That's fine. As it cools, this dome will contract a bit.) When finished baking, the cake should feel spongy, springing back slightly upon being lightly touched. A toothpick, inserted in the center, comes out free of cake. The toothpick is the last test for this cake before you judge if it is finished baking and remove it from the oven. If in doubt, baking 5 to 7 more minutes will not harm the cake.
Remove spongecake from the oven, and immediately turn it upside down, positioning the edges of the pan on the inverted glasses. This position, defying gravity, allows the cake to maintain its spongy structure without deflating while cooling. Cool the cake for 2 hours; then turn the pan right side up, and place it on the counter. Though the cake is cool to the touch, let it sit for at least 1 more hour to cool the inside completely. (The cake's structure is less fragile when cool, making removal easier.) To release the spongecake and maintain its shape perfectly, remove it from the pan as follows: Carefully loosen the cake crust that is stuck to the top rim of the pan; insert a thin metal spatula down 1/8 inch, and loosen and free the crust all around the cake. Then tilt and rotate pan, tapping it gently on the counter to free the sponge from the metal sides. Tap more if it is not completely released. Finally, release the spring mechanism, and slowly remove the rim. (You'll notice the cake seemed to have had a slipcover because a thin, crusty layer of cake remains on the springform rim; it's delicious, too.) Now tilt the cake on its side, and gently tap the metal bottom on the counter. Rotate the cake as you tap until the removable bottom appears free. Additional tapping may be necessary before it comes off completely. Patience will ensure a perfectly shaped cake.
For the filling: Pour the half and half into a 1 1/2 quart heavy bottomed saucepan, add the 1/2 cup sugar, and stir to combine. In a 1 quart bowl whisk the egg yolks and the remaining 1/2 cup sugar to combine. Add the flour, and whisk to combine. Bring the half and half mixture just to a boil over medium heat. Remove, and pour half of the hot liquid over the yolk mixture, whisking to combine. Pour the yolk mixture into saucepan, and over medium heat bring it to a boil again, stirring constantly. When it is thick and smooth, remove it from the heat, pour it into a 3 quart mixing bowl to cool, and cover the surface with plastic wrap to prevent a crust from forming on the cream. (This bowl size is to allow room for folding in the whipped cream later.) Pierce the plastic with the tip of a knife to let steam escape and cool faster. Refrigerate.
For the glaze: Chop the chocolate into matchstick size pieces with a chef's knife on a dry cutting board. Place first the butter, then the chocolate pieces in the top portion of a 1 1/2 quart double boiler (or a 1 quart mixing bowl that fits snugly over a saucepan or another mixing bowl). Fill the bottom vessel half full with hot tap water (120130 degrees) and place the chocolate/butter bowl on top to melt. You may put the double boiler on the stove over a very low flame just to maintain the water's temperature while melting the mixture if you wish. Stir occasionally to blend until the mixture is smooth, shiny and liquid. Remove from the water, and set aside. Yield: One scant cup.
Assembling the Trifle: Split the spongecake evenly into three layers, each about 7/8-inch thick, using a 12inch serrated knife in a sawing motion. Lift the top two layers, one at a time, and set aside (this is easy because each layer is spongy and thick). Stir the vanilla and rum into the cold custard. Whip the heavy cream in a 1 1/2 quart mixing bowl to the Chantilly stage, and fold into the custard mixture. Place the bottom cake layer of cake in a 2 1/2 to 3 quart glass bowl, and cover it with half of the filling, spreading evenly with a rubber spatula. Center the middle layer on top, and cover it with the remaining filling. Center the last layer on top. Cover the dessert with plastic wrap and refrigerate.
Spoon 1 cup of the Chocolate Butter Glaze into a 1 quart bowl over a bowl of hot tap water (120 to 130 degrees). Stir until liquid and smooth. Spread the glaze over the top of the dessert evenly with a metal or rubber spatula. With a 2inch paring knife, scrape the tip of the blade firmly but gently down a bar of chocolate. If the chocolate's surface area is cool, you will get fine flakes; if it's slightly warm, the shapes will be very small corkscrews. Sprinkle Chocolate Flakes around the rim of the dessert. Refrigerate for up to 1 to 2 hours before serving.