Step 1: Prepare the Pastry Cream
Whisk sugar and egg yolks until slightly fluffy. Add cornstarch and flour to form a smooth paste. Heat milk until simmering, then slowly whisk into the egg mixture. Return to heat and cook until thick. Stir in butter, cover, and cool.
Step 2: Make the Biscoff Buttercream
Whip butter until light, mix in Biscoff spread, then gradually add cooled pastry cream in stages until smooth and fluffy. Chill for 30 minutes.
Step 3: Make the Cupcakes
Preheat oven to 175°C. Cream butter and sugars until fluffy, then mix in the egg. Alternate adding dry ingredients and milk. Mix vinegar and baking soda, fold into batter. Fill cupcake liners and bake for 25 minutes.
Step 4: Assemble the Cupcakes
Once cooled, core each cupcake and fill with Biscoff spread. Pipe Biscoff Cupcakes with Biscoff Buttercream frosting on top and decorate with cookies and extra spread.