For the filling, in a small saucepan, combine the 1 cup milk and the vanilla bean (if using). Heat over medium heat until bubbles appear at the edges; set aside. In a medium saucepan, combine the 1/3 cup granulated sugar and cornstarch. Stir in the 1/3 cup milk. Remove vanilla bean from milk in small saucepan and scrape seeds into cornstarch mixture. Discard pod. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. Gradually stir about 1 cup of the hot vanilla milk into the beaten egg yolks, stirring constantly. Return all the mixture to the saucepan. Cook and stir until bubbly. Cook and stir for 2 minutes more. Remove from heat. Stir in the 2 tablespoons butter until melted. Stir in 1 teaspoon vanilla (if using). Cover surface with plastic wrap and refrigerate for 2 to 24 hours.
For the cake, allow eggs, the 3/4 cup milk, and the 1/2 cup butter to stand at room temperature for 30 minutes. Meanwhile, grease and lightly flour a high-sided 9-inch baking pan; set aside. In a medium bowl, stir together cake flour, baking powder, and salt; set aside.
Preheat oven to 350 degrees F. In a large mixing bowl, beat the 1/2 cup softened butter with an electric mixer on medium to high speed for 30 seconds. Add 1 cup granulated sugar and 1 teaspoon vanilla. Beat for 4 to 5 minutes or until fluffy. Add eggs, one at a time, beating well after each addition. Alternately add flour mixture and the 3/4 cup milk to butter mixture, beating on low speed after each addition just until combined. Spread batter in the prepared pan.
Bake for 35 to 40 minutes or until a wooden toothpick inserted near center comes out clean. Cool cake in pan on a wire rack for 10 minutes. Remove cake from pan and invert cake onto a wire rack. Cool thoroughly.
For the glaze, in a small saucepan, combine whipping cream and instant coffee crystals. Bring to boiling. Remove pan from heat. Stir in chocolate until mixture is smooth and chocolate is melted. Stir in corn syrup and powdered sugar until smooth. Let stand for 10 minutes to cool and thicken.
Using a long serrated knife, split cake in half horizontally. Remove top. Place bottom layer on a serving plate. Spoon chilled filling onto bottom layer and spread to 1/2 inch from edge. Top with remaining cake layer, cut side down. Spoon glaze onto center of cake. Spread evenly on cake, letting some run down sides. Refrigerate for 4 to 24 hours. Let stand at room temperature for 30 minutes before serving.