Place 1 1/2 cups milk and the vanilla into a medium saucepan. Cook over high heat, stirring often until it just starts to simmer. Remove from heat.

In a medium bowl, whisk remaining 1/2 cup milk, egg yolks, sugar, and cornstarch.

While whisking vigorously, slowly pour the hot milk into the egg mixture.

Return mixture to the saucepan and cook over high heat, whisking constantly, until it thickens and comes to a boil.

Pass through a strainer and into a bowl. Place plastic wrap directly on top of the custard to prevent a skin from forming.

Cool to room temperature then place in the refrigerator for a couple of hours to set.

Store in an airtight container in the fridge for up to a week. Do not freeze.

Ensure pastry cream and butter are at room temperature. Whip pastry cream by hand until smooth.

Using a stand mixer or hand mixer, beat the butter on high until pale and fluffy (3 mins). Add the room temperature custard 1 Tbsp at a time, incorporating well after each addition. Add powdered sugar or more vanilla if desired. Beat for 2-3mins until smooth and fluffy.

Store in an airtight container in the fridge for up to 2 weeks or freeze for up to 3 months. Allow buttercream to come to room temperature and re-whip before using.