Prepare 2 round layers (8 or 9") according to package directions and then cool completely.
If desired slice "dome" off top of layers to make them more flat.
Pastry Cream
Beat egg yolks in small bowl and then whisk in cornstarch and set aside.
Combine milk, sugar and vanilla in a heavy saucepan and heat over medium heat, stirring until sugar is dissolved and mixture comes to a simmer.
Set milk mixture off heat and allow to cool for about 10 minutes.
Stir ½ cup of milk mixture into egg yolk mixture very gradually, whisking constantly.
Add egg yolk mixture to milk mixture gradually and return to medium heat, whisking constantly until thickened and continue to cook for 1-2 minutes, reducing heat to medium-low.
Remove from heat and stir in butter until melted and thoroughly mixed.
Pour pastry cream into a bowl and lay plastic wrap directly on surface. Refrigerate for 4 hours or up to a day before spreading on bottom cake layer and then topping with 2nd cake layer.
Chocolate Ganache
Heat cream over medium to medium high heat just until it begins to boil and then remove from heat.
Add chocolate and whisk until chocolate is melted and mixture is smooth.
Stir in corn syrup and whisk until smooth.
Cool 3-4 minutes before pouring over cake, allowing to drizzle down sides.
Notes
Any buttery yellow cake can be used, and if desired, one layer can be split horizontally to create two layers.