Preheat Oven & Prepare Cake Tins:
Preheat your oven to 180°C (350°F). Grease and line two 20cm (8-inch) round cake tins with parchment paper.
Mix Dry Ingredients:
In a large mixing bowl, sift together plain flour, light brown sugar, baking powder, baking soda, and Speculoos spice (optional). Add the crushed Biscoff biscuits (optional) and mix well.
Add Wet Ingredients:
Add unsalted butter, eggs, milk, and vanilla extract to the dry ingredients. Beat with a hand mixer or stand mixer on low speed until combined, then increase speed to medium and beat until smooth.
Bake the Cake:
Divide the batter evenly between the two prepared cake tins. Level the tops and bake for 30-35 minutes, or until a skewer inserted into the center comes out clean. Let the cakes cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
Make the Cookie Butter Frosting:
Beat unsalted butter in a mixing bowl until creamy. Add Biscoff spread, vanilla extract, and sea salt, and mix until smooth.
Gradually add icing sugar, mixing on low speed until fully incorporated. Increase speed to medium and beat until smooth and fluffy. Add hot water to adjust the consistency as needed.
Assemble the Cake:
Once the cakes are completely cool, level the tops if necessary. Place the first cake layer on a serving plate. Spread a generous amount of cookie butter frosting on top.
Place the second cake layer on top and repeat with more frosting.
Decorate the Cake:
Microwave the remaining Biscoff spread for 20-30 seconds until it’s smooth and runny. Drizzle this over the top of the cake, letting it drip down the sides.
Spread any remaining frosting over the top and sides. Decorate with Biscoff biscuits.