"A rich and delicious chocolate cake with a delicate custard filling inside. You can dust the top with cocoa powder for a simple delight or Super Delicate Chocolate Buttercream Frosting for an elegant one."
Preheat oven to 375 degrees F (190 degrees C). Grease a 9 inch round cake pan, and line the bottom with parchment paper. Grease and flour the paper.
In the top of a double boiler, heat chocolate, stirring occasionally, until chocolate is melted and smooth. Remove from heat and allow to cool to lukewarm. Sift together the cocoa, flour, baking powder and cornstarch. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the vanilla and milk powder. Beat in the egg yolks one at a time. Beat in the flour mixture alternately with the melted chocolate.
In a large glass or metal mixing bowl, beat egg whites until foamy. Gradually add 2 tablespoons sugar and 1/2 teaspoon salt, continuing to beat until stiff peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain. Pour batter into prepared pan.
Bake in the preheated oven for 50 to 60 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.
When cake is cool, slice horizontally into 3 layers. Brush layers with 1/2 cup rum and let stand 60 minutes. Place the bottom layer of cake on serving dish and spread with 1/2 of the custard filling. Repeat layers and top with last layer of cake.
To make the Custard Filling: In a saucepan, combine 3/4 cup sugar and 1/2 cup cornstarch. Stir in milk. Cook, stirring constantly, until mixture thickens. Place egg yolks in a medium bowl. Slowly whisk 1/2 cup of hot milk mixture into egg yolks. Slowly pour the egg yolk mixture back to remaining milk mixture, whisking constantly. Cook, stirring constantly, for 5 minutes, or until thickened. Remove from heat and stir in unsalted butter and 2 tablespoons rum.