Heat the Milk: Split vanilla beans and scrape out seeds. Heat the milk and vanilla beans (or just the milk if using extract) in a saucepan until it almost simmers. Remove from heat at the first bubbles.

Temper the Eggs: Whisk egg yolks, sugar, and cornstarch until pale. Slowly add 1/4 cup of hot milk while whisking to avoid cooking the eggs. Continue until all milk is incorporated.

Cook the Custard: Pour mixture back into the pan through a strainer to catch any cooked egg bits. Cook over medium heat, stirring constantly until thick. Remove from heat and stir in vanilla extract if using.

Cool the Custard: Place custard in a bowl, cover with plastic wrap touching the surface to prevent a skin, and refrigerate for at least 2 hours. You can make it a day ahead.

Preheat Oven: Set the oven to 350°F. Line and grease an 8” or 9” springform pan.

Mix Dry Ingredients: Whisk together flour, baking powder, baking soda, and salt.

Cream Butter and Sugar: Beat butter for 30 seconds, then add sugar and vanilla seeds. Beat until light and fluffy. Scrape the bowl as needed.

Add Wet Ingredients: Mix in eggs one at a time, then add vanilla extract and sour cream.

Combine Ingredients: Add half the dry ingredients, mix on low. Add buttermilk, then the rest of the dry ingredients. Mix until just combined to avoid a dry cake.

Bake: Spread batter in the pan. Bake for 4045 minutes, covering with foil if it browns too quickly. It’s done when a toothpick comes out clean. Cool completely.

Fill the Cake: Once the cake is cool, poke holes using a dowel or chopstick. Fill half the pastry cream in a piping bag and pipe into the holes. Spread the rest on top.

Chill: Refrigerate the cake for at least 30 minutes.

Caramelize: Sprinkle a thick layer of sugar on top. Use a torch to toast the sugar until crispy. Let it cool to harden.

Serve: Slice and enjoy!