Preheat the oven to 350 degrees F. Grease and flour a 9-inch springform pan. Line the bottom of the pan with parchment paper or wax paper. Grease the paper and set aside.
In a mixing bowl, beat the egg yolks on high speed about 5 minutes, or until thick and lemon colored. Add the vanilla. Reduce the speed to low and gradually beat in 1 cup of the sugar. Continue beating until the mixture thickens slightly and doubles in volume, about 5 minutes. In a small bowl, combine the cake flour, cocoa, and baking powder. Sift 1/3 cup of the flour mixture over the egg yolk mixture and fold it in until combined. Repeat with 1/3-cup measures of the remaining flour mixture until it is all incorporated. Blend in the butter.
In a mixing bowl, beat the egg whites and cream of tartar on speed until soft peaks form, about 2 minutes. Gradually add the remaining ¼ cup of sugar to the egg whites, beating on high speed until stiff peaks form. Gently fold 1 cup of the beaten egg white mixture into the egg yolk mixture to lighten the batter, then fold the whole yolk mixture into the egg white mixture. Pour into the prepared pan.
Bake for 30 to 35 minutes, or until the cake springs back when lightly touched. Cool on a wire rack for 10 minutes. Remove the pan and cool completely.
Cut the cake in half horizontally, forming 2 equal layers. Place one layer on a serving plate and spread with Blackout Cream. Top with the remaining cake layer. Frost the top and sides with the remaining Blackout Cream. Sprinkle the cake with the chocolate wafer crumbs.
In a medium saucepan over low heat, combine 1 ½ cups of water, the sugar, and corn syrup. Stir in the chocolate and cook, stirring constantly, until the chocolate is melted, about 2 to 3 minutes.
In a small bowl, combine ½ cup of water and the cornstarch and whisk until the cornstarch is dissolved. Add the dissolved cornstarch paste and the heavy cream to the chocolate mixture and bring to a boil, stirring constantly. Boil for 1 minute. Remove from the heat and whisk in the butter.
Transfer the mixture to a bowl. Cover the surface with plastic wrap and chill for 1 hour before serving
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