Prepare the Oven & Cupcake Tin: Preheat your oven to 180°C (350°F). Line a 12-cup muffin tin with cupcake liners.
Mix the Cupcake Batter:
In a large mixing bowl, beat together the unsalted butter and light brown sugar until fluffy.
Add the eggs, one at a time, and continue to beat until well combined.
Sift the self-raising flour and gently fold it into the mixture until fully combined. If using, add the vanilla extract.
Fill the Cupcake Cases: Using an ice cream scoop or spoon, evenly divide the batter into the cupcake liners. Fill each liner about two-thirds full.
Bake the Cupcakes: Place the tin in the oven and bake for 17-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. The cupcakes should be golden and springy to the touch.
Make the Buttercream:
In a separate bowl, beat the butter until soft and smooth. Gradually add the icing sugar and continue to beat until fluffy.
Add the Biscoff spread and mix until smooth. If the buttercream is too thick, add 1-2 tbsp of boiling water to adjust the consistency.
Cool and Decorate: Allow the cupcakes to cool completely on a wire rack before frosting. Once cooled, pipe the Biscoff buttercream on top of each cupcake. Drizzle with melted Biscoff spread and sprinkle with crushed Biscoff biscuits for decoration.