Vanilla Pastry Cream:
Heat the whole milk with vanilla bean seeds and pods until nearly boiling, then remove from heat to infuse.
Whisk egg yolks, sugar, and cornstarch until pale. Temper eggs by whisking in 1/4 cup of hot milk. Gradually add the remaining milk, whisking continuously.
Strain back into the saucepan and cook over medium heat, stirring until thickened.
Remove from heat, add vanilla extract if using, cover with plastic wrap, and refrigerate for at least 2 hours.
Cake Batter:
Preheat oven to 350ºF (325ºF if using convection). Prepare an 8 or 9-inch springform pan lined with parchment paper.
Mix flour, baking powder, baking soda, and salt.
Beat butter until creamy, add sugar and vanilla, and beat until fluffy. Add eggs one at a time, mixing after each addition.
Mix in the sour cream. Alternate adding dry ingredients and buttermilk, mixing until combined.
Pour batter into the pan and bake for 40-45 minutes, checking doneness with a toothpick.
Cool completely on a wire rack.
Assembly:
Poke holes in cooled cake and fill with half the pastry cream using a piping bag.
Spread remaining pastry cream on top. Refrigerate for at least 30 minutes.
Sprinkle a thick layer of sugar and use a kitchen torch to caramelize until the sugar forms a crispy crust. Let cool slightly before serving.