Vanilla creme pat:

20oz milk (2.5 cups)
1 vanilla pod, split in half and seeds scraped out and reserved
6 egg yolks
100g powdered sugar
50g cornstarch
50g butter

Pour the milk into a pan with the vanilla seeds and pod, heat over low until simmering. Remove from heat.

Whisk the egg yolks, sugar, and cornstarch together until pale and creamy.

Remove the vanilla pod from the warm milk.

Stir the warm milk slowly into the egg mixture. Return to the heat, cooking over low heat until 4-5 minutes and whisking constantly until very thick.

Turn off heat, mix in butter until melted and incorporated. Transfer to a bowl, cover with plastic wrap to prevent a skin forming, refrigerate.

Vanilla buttercream:

2 sticks butter, softened
500g powdered sugar
2 tsp vanilla extract
1-2 tbsp heavy cream

Whip the butter in a large bowl.

Add 2 cups powdered sugar, mix until combined.

Add vanilla and 1 cup sugar. Mix until combined.

Add milk if needed to thin, remaining cup of sugar. Mix until thick and creamy, and until it can hold a point.