Heat the milk. Add the milk, half the sugar, and vanilla extract (if using) to a saucepan. Heat over medium until it just begins to steam and is close to a boil.
2 cups Milk, 1/3 cup Sugar, 1 tsp Vanilla extract
Prepare the yolks. While the milk heats, whisk the egg yolks, remaining sugar, and cornstarch in a large heatproof bowl until smooth and pale. Add a small splash of the warm milk and whisk until fully combined — this loosens the mixture and makes tempering easier.
4 Egg yolks, 5 tbsp Cornstarch
Temper the eggs. Remove the milk from the heat. Slowly pour it into the yolk mixture in a steady stream, whisking constantly. Don't rush this — pouring too fast can scramble the eggs.
Cook until thick and bubbling. Return the mixture to the saucepan. Cook over medium heat, whisking constantly, until it thickens and begins to bubble. Once bubbling, lower the heat and keep whisking for 30–60 seconds to cook out any raw starch taste.
Whisk off heat. Remove from heat and continue whisking for another 30–60 seconds. This slightly loosens the cream so it sets smooth and firm, not stiff or lumpy.
Add the butter. Stir in the butter until fully melted and incorporated.
1 tbsp Unsalted butter
Add the Biscoff. Add the Biscoff spread and give it a minute to warm up from the residual heat, then stir until fully incorporated and smooth.
1/2 cup Biscoff spread
Taste and adjust. Taste and add more sugar or a pinch of salt if needed.
Cool properly. Transfer to a bowl and press plastic wrap directly onto the surface. Let cool to room temperature, then move to the fridge and seal tightly. You can remove the plastic wrap at this point or leave it on. Keeps for 3–4 days.