Step 1: Prepare the Cake

1. Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans.

2. In a bowl, whisk together flour, baking powder, and salt. Set aside.

3. In another bowl, cream the butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in the vanilla.

4. Gradually add the dry ingredients alternately with milk, starting and ending with the dry mixture.

5. Divide the batter evenly between the pans and bake for 25-30 minutes, or until a toothpick inserted comes out clean.

6. Let the cakes cool completely on a wire rack before assembling.

Step 2: Make the Custard Filling

1. In a saucepan, heat the milk over medium heat until it begins to steam. Do not boil.

2. In a bowl, whisk together egg yolks, sugar, and cornstarch until smooth. Slowly pour the hot milk into the egg mixture while whisking constantly.

3. Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens.

4. Remove from heat, stir in vanilla, and transfer to a bowl. Cover with plastic wrap, pressing it directly onto the surface to prevent a skin from forming. Chill for at least 1 hour.

Step 3: Prepare the Chocolate Ganache

1. Heat heavy cream in a saucepan over medium heat until it just begins to simmer.

2. Pour the hot cream over the chopped chocolate in a heatproof bowl. Let sit for 1 minute, then stir until smooth and glossy.

3. Allow the ganache to cool to a spreadable consistency.

Step 4: Assemble the Cake

1. Place one cake layer on a serving plate. Spread the custard filling evenly over the top.

2. Place the second cake layer on top of the custard.

3. Pour the ganache over the top layer, spreading it evenly and allowing some to drip down the sides.

4. Chill the assembled cake for 30 minutes to set.