Preheat the oven to 170°C (340°F). Grease a 9×13-inch cake tin with cake release spray and line with baking paper, leaving overhanging edges for easy removal.
Prepare the dry ingredients: In a large mixing bowl, combine the flour, sugar, cinnamon, baking powder, bicarbonate of soda, and Biscoff biscuit crumbs. Stir until well mixed.
Mix wet ingredients: In a separate bowl, whisk together the softened butter, eggs, milk, and vanilla extract until smooth and fluffy.
Combine: Gradually fold the wet ingredients into the dry mixture. Use a spatula to combine, being careful not to overmix.
Bake: Pour the batter into the prepared cake tin and level it out. Bake for 30-35 minutes or until the cake is golden, springy to the touch, and a skewer inserted comes out clean.
Cool: Allow the cake to cool in the tin for 10 minutes, then lift it out using the baking paper and cool completely on a wire rack.
Prepare the buttercream: In a bowl, combine the softened butter, icing sugar, Biscoff spread, and vanilla extract. Beat on low speed to combine, then increase the speed and add milk or cream to reach a smooth, spreadable consistency.
Frost the cake: Spread the Biscoff buttercream over the cooled cake using an offset spatula. Garnish with chopped Biscoff biscuits and drizzle with melted Biscoff spread if desired.
Serve: Slice the cake into squares and serve.
