Make Filling First: Bring milk, 1 tablespoon of sugar and vanilla to a simmer in large heavy saucepan; stir to dissolve sugar. Meanwhile, stir together 3/4 cup of sugar, cornstarch and salt in a small mixing bowl; set aside.Beat the eggs in a large mixing bowl with an electric mixer on medium speed till fluffy and lemon colored, about 3 minutes. Stir in the corn starch mixture.
Whisk 1/2 cup of hot milk mixture into the egg mixture, then add the egg mixture back to the milk in the saucepan. Cook, stirring constantly, over medium heat until thick, about 5 minutes. When mixture starts to boil, continue boiling for another minute.
Remove from heat and add room temperature butter and rum (if using).
Pour into a bowl and cover with a piece of plastic wrap or parchment to prevent a skin from forming.Chill until cakes are baked and completely cooled.Make Cakes: Preheat oven to 350 degrees. Spray 2 9-inch cake pans with flour- added cooking spray and line with parchment paper rounds.Now you will make the golden and chocolate cake batters. To get started, sift together the dry ingredients for the golden cake and the dry ingredients for the chocolate cake and put them in separate bowls.Make the golden batter first. In a large bowl, beat eggs and sugar on high speed until light and fluffy, 8 minutes. Gently fold sifted flour mixture (for the golden cake, of course) into eggs in two additions, making sure it gets thoroughly mixed. Stir in milk, oil and vanilla.
Pour batter into one of the prepared cake pans and put it in the oven to bake for 25 minutes.Meanwhile, make the chocolate batter. Wipe the bowl clean and beat the eggs and sugar for 8 minutes. Gently fold sifted flour mixture into eggs in two additions. Fold in coffee, oil, and vanilla until thoroughly mixed.
Pour batter into other the other cake pan and bake the chocolate cake for 15 to 18 minutes or until it springs back when touched.
Transfer cakes to a rack; cook 10 minutes. Run a knife around edges to loosen, then invert cakes onto plates.
Remove parchment and invert again onto the rack so the cakes are right side up. Cool completely.To assemble, split golden cake into two layers and place the bottom half cut side up on a cake platter. Top with half of the cold filling (about 1 1/4 cups) spreading it evenly to the edges of the cake. Carefully place the chocolate layer on top and spread with remaining filling. Top with second golden layer, cut side down; press lightly to stabilize layers. Chill cake for about 15 to 20 minutes.For the glaze, place finely chopped chocolate in a small bowl. Boil cream and corn syrup, pour over chocolate and whisk until smooth.
Let it cool for about 10-20 minutes or until it thin enough to pour, but not so thin that it will flow all over the place when you pour it.
