Pre-heat your oven to 160C Fan/350F/Gas Mark 4, and grease and line two 8" cake tins (that are at least 2" deep)
Make the sponge by melting the butter, golden syrup, light brown soft sugar and black treacle together in a pan on a low heat. Once it is all melted and combined, leave to cool for 10 minutes
Whisk the eggs and milk together in a large bowl
Slowly pour the butter and syrup mixture into the eggs, whisking as you pour, until everything is combined
Add the self raising flour. ground ginger and mixed spice, and gently fold or whisk in
Divide the mixture between the tins, use scales for accuracy
Bake them for 40-45 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 ginger buttercream mix the butter on it's own for a few minutes, then add the ground ginger, vanilla extract, milk and icing sugar and mix until smooth. Use an electric whisk or a stand mixer with the paddle attachment for best results
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 broken up ginger nut biscuits and crystallised ginger
Leftovers will keep in an airtight container in a cool place for 2-3 days