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
Make the sponge by mixing the butter and light brown sugar in a large bowl until fluffy, ideally using an electric mixer
Add the Biscoff spread and mix in
Add the eggs and milk, and whisk until fully incorporated
Gently whisk in the self raising flour, and whisk in until you can't see any flour anymore
Divide the mixture between the tins, use scales for accuracy
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
To make the buttercream, use an electric mixer to mix together the butter and icing sugar until they start to combine, then add the Biscoff spread and milk and mix until smooth. You can add more milk if the buttercream is too stiff
If the cakes have domed on top, 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