Preheat oven to 180°C (160°C fan).
Grease and line a 9 x 13-inch rectangular baking tin with parchment paper.
Place flour, sugar, crushed biscuits, baking powder and bicarbonate of soda in a bowl and combine.
Add butter, eggs, milk and vanilla to the dry ingredients.
Beat together with an electric whisk until combined to make a batter.
Pour into the prepared tin, and bake for 30 minutes or until cooked through.
Leave to cool in its tin.
Whilst the cake is cooling make your frosting.
Add butter, vanilla, icing sugar and crushed Biscoff to a bowl and blend together until the frosting is spreadable.
Spread the icing over the cooled cake - a spatula is good for this.
Sprinkle with crushed Biscoff biscuits.