Put the margarine, sugar, banana and Lotus Biscoff spread in a large bowl and whisk together.
Add the eggs and beat again.
Fold in the flour and baking powder.
Divide the batter equally between two bowls, then add the cocoa to one batch and whisk through.
Grease the loaf tin and line with baking paper.
Spoon in the mixtures in alternating dollops.
Swirl gently with a skewer.
Bake for 1 hour until golden brown on top and fully cooked through. You can check this by pushing a skewer into the centre of the loaf. If it comes out clean, the cake is ready.
Place on a rack to cool completely.
Make the biscoff frosting
Put the butter, icing sugar, salt, Biscoff spread and milk in a large bowl and whisk until pale and creamy.
Decorate your banana Biscoff cake
Use a piping bag with star nozzle to pipe the buttercream over the top of the cake in big swirls. Or spread evenly to the edges with a palette knife or silicone spatula.
Lightly warm the Biscoff smooth spread for about 5-10 seconds in the microwave, just enough to make it runny, then drizzle over the top of the cake.
Break some of the Biscoff caramelised biscuits into halves and crush others.
Sprinkle the Biscoff crumbs over the frosting, then push in the larger biscuit pieces and banana chips.