Preheat the oven to 180oC. Line a Swiss roll tin with parchment paper or 2 small cake tins.
Sieve the flour and baking powder into a bowl. Pop in the soft margarine and sugar. Crack the eggs in and pour in the milk.
Using an electric mixer, beat the ingredients for 1-2 minutes until the mixture is all smooth and combined. Be careful not to over-mix, this is what makes the cake tough and dry.
Pour into the tin, smooth it out with a spatula and bake for 25-30 minutes.
While the cake bakes make the icing. Whip the room-temperature soft butter until it is silky and smooth. Gradually pour in the icing sugar and mix. Add the milk little by little and continue to whip for about 5 minutes until incredibly smooth.
Remove the cake from the oven and let it cool. Cut into squares 12 or 24 squares. Cut each square in half and fill with a teaspoon of biscoff spread.
Pop the top layer back on and pipe on the coffee buttercream.
Serve and enjoy.