Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line two 8" cake tins (that are at least 2" deep)
In a mixing bowl, stir together the light brown soft sugar, self raising flour and baking powder
In another mixing bowl or large jug, whisk together the vegetable oil, milk, vanilla extract, biscoff spread and vinegar. I find it's easier to add the biscoff first, then whisk the oil into it bit by bit, then add the other ingredients
Make a well in the centre of the flour mixture, and pour in the oil and milk mixture, whisk them together gently with a hand whisk until smooth
Divide the mixture between the tins, use scales for accuracy if you like
Bake them for 35-40 minutes or until a skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or remove from the tin and place on cooling racks
Make the buttercream by mixing together the dairy free spread and icing sugar, then add the biscoff spread and milk, and mix until smooth. You can do this by hand or for best results use an electric whisk
If the cakes have domed on top, you can level them off with a cake leveller or a serrated knife
Put one of the sponges on your plate or cake stand and pipe or spread some of the buttercream onto it
Add the other sponge on top and pipe or spread the remaining buttercream on top
Decorate with the melted Biscoff spread (microwave for 20 seconds to melt it) and the Biscoff biscuits. The biscuits will go soft after several hours in contact with the buttercream so add them later if you're not serving the cake straight away
Leftovers will keep in an airtight container in a cool place for 2-3 days