Preheat the oven to 325°F. Butter and flour a 9" round cake pan (make sure you use one that is at least 2" deep)--I like to line it with parchment as well and then grease it.
Whisk together the flour, baking powder, and salt.
Add the butter and milk to a small saucepan and bring to a simmer, stirring until the butter melts. Remove from the heat and set aside to cool slightly.
In the bowl of a stand mixer, beat the eggs and sugar together until very pale and fluffy, about 5 minutes. Add the hot butter/milk mixture slowly, beating constantly. Fold in the dry ingredients and mix until just combined.
Pour the batter into your prepared pan and bake for 30 to 35 minutes, until a tester inserted in the center comes out clean.
Remove from the oven, let cool in the pan for 10 minutes, then flip onto a wire rack to finish cooling while you make the filling.
To make the filling: Whisk together the egg yolks, sugar, and flour in a medium bowl.
In a small saucepan, bring the milk and 3/4 cup of the cream to a simmer, then slowly whisk 1/2 cup of the hot cream into the egg yolk mixture. Pour the egg yolk mixture back into the saucepan, whisking constantly, and cook over low heat until the pastry cream thickens (about 3 minutes). Remove from the heat and stir in the vanilla and 2 ounces of the chocolate, stirring until the chocolate melts. Let the pastry cream cool to room temperature, then chill it until cold before filling the cake.
Make the ganache by bringing the remaining 1/2 cup of cream to a simmer. Remove from the heat and stir in the remaining 8 ounces of chocolate until smooth.
To assemble the cake, slice the cake in half horizontally into two even layers. Spread the pastry cream in between the two layers. Pour the chocolate ganache over the top, spreading it to the edges.