Preheat the oven to 200C/180C Fan/Gas 6. Line/grease an 8" square tin.
In a large bowl, or the bowl of a stand mixer, combine the butter, Biscoff spread and sugar for 3-5 minutes on a medium/high speed. Add the eggs and mix again.
Add the flour, baking powder and baking soda. Fold together before adding the milk to thin the cake batter a little. Pour the mix into the lined tin and bake for 25-30 minutes, until a skewer inserted comes out clean. Let cool.
Make the frosting by combining the butter and Biscoff spread on a high speed with an electric mixer for several minutes. Add half of the icing sugar and beat to combine. Continue to add more icing sugar until you reach the desired consistency. Spread over the cake. Decorate with Biscoff biscuits and serve!