Bake 2 cakes according to package instructions in a 3-inch deep cake pan. Once baked, remove from the oven and cool on a wire rack. Once cooled, take a long serrated knife and slice each cake in half to form 4 separate disks.
Pour the milk, 1/4 cup sugar and the vanilla bean in a saucepan over medium heat. Combine the egg yolks and the remaining 1/4 cup sugar in a bowl and whisk until light in color. Add the flour, vanilla extract and the salt to the egg yolks and mix to combine. When the milk just begins to boil, remove from heat. Very slowly temper the hot milk into the yolk mixture, stirring constantly. When about half of the milk has been added, pour all of the yolk mixture into the saucepan over medium heat. Using a whisk, mix as it heats, making sure to reach all of the corners of the pan when stirring. Bring the mixture to a boil. Let it boil for about 1 minute, stirring constantly. The mixture will be thick. Remove from heat and add the butter. Strain the pastry cream through a fine strainer. Put it into a bowl and cover directly with plastic wrap to prevent a skin from forming on the cream. Refrigerate to chill for at least 1 hour before using.
In a small saucepan, over low heat bring the cream to a boil. Add the chocolate to a small bowl and pour in the hot cream. Stir until smooth, then allow to cool slightly.
Spread cooled pastry cream on the top of 3 of the cake layers. Put each disk of cake on top of each other, leaving the last cake disk untouched. Press the cake firmly. Put the cake on a wire rack with a sheet tray underneath. Pour the slightly cooled ganache on top of the cake being sure the ganache covers the whole cake. Allow to cool in the refrigerator or at room temperature. Put any remaining pastry cream in a piping bag with a star tip and use to decorate the top. Slice the cake and serve.