To make the cake: Butter a jelly roll pan (15 1/2 by 10 1/2 by 1-inch), line it with wax paper and butter the paper. In the top of a double broiler set over hot water, stir the chocolate with the coffee until it is just melted and let the mixture cool. In a bowl using an electric mixer, beat the egg yolks until they are combined, beat in 1/2 cup of the sugar, a little at a time, and beat the mixture until it ribbons when the beater is lifted.
In another bowl, using an electric mixer, beat the egg whites with the salt until they are frothy. Add the cream of tartar and beat the whites until they hold soft peaks. Beat in the remaining 1/4 cup sugar, a little at a time, and beat the whites until they hold stiff peaks.
Add the chocolate mixture to the egg yolk mixture and combine well. Stir in 1/4 of the whites and fold in the remaining whites gently but thoroughly. Spoon the batter into the pan, spread it evenly with a spatula and bake the cake in a preheated oven at 350 degrees for 15 to 20 minutes, or until a cake tester comes out clean. Transfer the pan to a rack and let the cake cool completely, covered with dampened paper towels. Remove the towels and run a knife around the sides of the pan to release the waxed paper. Cover the top of the cake with waxed paper and a baking sheet, invert the cake onto the baking sheet and peel off the waxed paper on top.
To make the praline powder: In a heavy saucepan bring to a boil the sugar with the cream of tartar and water over moderately high heat, stirring and washing down any sugar crystals clinging to the sides of the pan with a brush dipped in cold water. Cook the syrup over high heat without stirring, swirling the pan until it is a light caramel. Add the almonds and swirl the pan until the nuts are coated with the caramel and begin to make a popping sound. Pour the mixture onto an oiled marble surface or jelly roll pan and let cool until it is hard. Transfer the praline to a cutting board, chop it coarse and use a food processor or blender to pulverize it in batches. Store the praline powder in an airtight container.
In a bowl, using an electric mixer, beat 1 1/2 cups of the praline powder with the ice cream until it is just soft enough to spread, and spread it over the cake, leaving a 1-inch border. Starting with a long side, roll up the cake jelly roll fashion, lifting it with the waxed paper and finishing with the seam side down. Transfer it to a platter and freeze it for at least 2 hours, or until the ice cream is frozen.
In a chilled bowl, beat the cream with the confectioners' sugar until it just holds soft peaks. Spoon 3/4 of it into a pastry bag fitted with a ribbon tip and pipe it over the cake. Spoon the remaining cream to a pastry bag with a 1/4 inch plain round tip and pipe it onto the ends of the cake. Or spread all the cream over the cake with a spatula. Freeze the cake for 1 hour more. Sprinkle the cake with the remaining 1/2 cup praline powder and garnish it with the caramel cutouts, if desired.
To make the caramel cutouts: Coat 6 metal "open" cookie cutters with the oil and arrange them 2 inches apart on a well-oiled jelly roll pan. In a heavy saucepan bring to a boil the sugar with the cream of tartar and the water over moderately high heat, stirring and washing down any sugar crystals clinging to the sides of the pan with a brush dipped in cold water. Cook the syrup over high heat without stirring, swirling the pan, until the syrup is caramel-colored. Let it cool for 1 minute and spoon enough of it carefully into each cookie cutter to measure 1/8-inch. Let the cutouts cool until they are hard and release them from the pan and cutters with the tip of a knife. Arrange them fancifully on the ice cream log.
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