Lightly coat the insides of 3 (9 by 1 1/2-inch) cake pans with melted butter. Line each pan with parchment paper, then lightly coat the parchment with more melted butter. Set aside.
Preheat the oven to 325 degrees F.
Heat 1-inch of water in the bottom half of a double boiler over medium heat. Place remaining butter and 8 ounces semisweet chocolate in the top half of the double boiler. Tightly cover the top with film wrap. Allow to heat for 10 to 12 minutes. Remove from heat, stir until smooth, and hold at room temperature.
Place the egg yolks and 3/4 cup sugar in the bowl of an electric mixer fitted with a paddle. Beat on high until slightly thickened and lemon-colored, about 4 minutes. Scrape down the sides of the bowl and beat on high for an additional 2 minutes.
While the egg yolks are beating, whisk 4 egg whites in a large stainless steel bowl until stiff but not dry, about 3 to 4 minutes. Using a rubber spatula, fold the melted chocolate mixture into the beaten egg yolk mixture. Add a quarter of the beaten egg whites and stir to incorporate, then gently fold in the remaining egg whites. Divide the batter between the prepared pans, spreading evenly, and bake in the preheated oven until a toothpick inserted in the center comes out clean, about 25 to 30 minutes. Remove the cakes from the oven and allow to cool in the pans for 15 minutes. (During baking, the surface of the cakes will form a crust; this crust will normally collapse when the cakes are removed from the oven.) Invert 1 of the cakes onto the bottom of a springform pan. Invert the other 2 cakes onto cake circles. Remove the parchment paper and refrigerate cakes for 30 minutes.
To prepare the chocolate and strawberry mousse, reserve the 12 best-looking strawberries to decorate the top of the cake. In a food processor fitted with a metal blade, puree 4 ounces of strawberries (12 medium-sized berries should yield 1/2 cup of puree). Set aside until needed. Refrigerate the remaining berries until needed (that includes the 12 berries for decoration) Heat 1-inch of water in the bottom half of a double boiler over medium heat. Place 16 ounces semisweet chocolate, the white chocolate and the strawberry puree in the top half of the double boiler. Tightly cover the top with film wrap. Allow to heat for 12 to 14 minutes. Remove from the heat and stir until smooth. Transfer to a stainless steel bowl, using a rubber spatula to remove all of the melted chocolate mixture. Keep at room temperature until needed. In the bowl of an electric mixer fitted with a balloon whip, whisk the 6 egg whites on high until soft peaks form, about 2 minutes. Continue to whisk on high while gradually adding 2 tablespoons sugar. Whisk until stiff but not dry, about 30 seconds. Set aside at room temperature until needed.
Using a hand-held whisk, whip the heavy cream in a well-chilled stainless steel bowl until stiff. Fold a quarter of the egg whites into the melted chocolate mixture, then fold in the whipped cream. Now fold in the remaining egg whites. Set aside at room temperature until needed.
Assemble the springform pan. Spread 1/2 cup of chocolate strawberry mousse onto the cake layer in the assembled springform pan. Arrange 1/2 the amount of reserved strawberries (not including the 12 for decoration), stem side down, into the mousse. The berries should be arranged in 2 rings: the first ring being 3/4-inch from the outside edge of the cake, and the second, inside ring, 3/4-inch away from the first. Distribute 3 cups of mousse over the berries, being careful to keep the berries in position. Holding the pan by the top rim, gently but firmly tap the bottom of the pan 2 to 3 times on your work surface (this will eliminate air pockets). Position a cake layer on top of the mousse, then repeat the process used on the first cake layer (1/2 cup mousse, remaining strawberries, and 3 more cups mousse). Top the mousse with the remaining cake layer and gently press into position. Refrigerate the cake for 2 hours (do not freeze).
Refrigerate 1 cup of Semisweet Chocolate Ganache for at least 1 hour. Keep the remaining ganache at room temperature until needed.
Remove the sides from the springform pan (do not remove the bottom of the pan from the cake; this will make it easier to handle later). Use a cake spatula to smooth and fill in the sides of the cake with 2 to 3 tablespoons of room-temperature ganache. Evenly spread the remaining amount of this ganache over the top and sides of the cake. Refrigerate the cake for 1 hour.
Transfer the chilled cup of ganache to a pastry bag fitted with a large-sized star tip. Remove the cake from the refrigerator. Pipe a circle of 12 evenly spaced stars along the outside edge of the top of the cake. Place a strawberry, stem side down, into each ganache star. Refrigerate the cake for 30 minutes.
Cut the cake with a serrated slicer, heating the blade of the slicer under hot running water before making each slice. Allow the slices to come to room temperature for 20 to 30 minutes before serving.
Semisweet Chocolate Ganache:
Heat the heavy cream, butter, and sugar in a 2 1/2-quart saucepan over medium-high heat. When hot, stir to dissolve the sugar. Bring the mixture to a boil. Place the chocolate in a stainless steel bowl. Pour the boiling cream over the chocolate and allow to stand for 5 minutes. Stir until smooth. Allow to cool to room temperature.