Preheat your oven to 160C Fan /180C and prepare a cupcake / muffin tin with 12 cases.
Cream together your softened butter and caster sugar until light and fluffy.
Add your eggs in one at a time, mixing in between each and add your vanilla extract.
Add your flour, baking powder and xanthan gum and mix until combined, then split your mixture equally into two separate bowls.
Mix together your cocoa powder and milk (it should be a paste, but add a little extra milk if too dry) and add to one of the bowls. Then, mix to combine briefly.
Spoon alternative dollops of the chocolate and vanilla mixture into your cases until two thirds full, then use a skewer to swirl the colours together. Bake for about 22 minutes until cooked through.
Allow to fully cool before coring the centre of each cupcake and filling with a tsp of chocolate spread. I use the wide end of a piping nozzle to do this, but a cupcake corer works best. Just ensure you don't go too deep so there's still sponge to support the filling.
For the buttercream, beat your softened butter in a large bowl until pale in colour, then add your icing sugar and mix once more until combined. Split your mixture into two separate bowls.
To one bowl add ½ tsp vanilla extract and to the other bowl add 25g cocoa powder. Mix both in thoroughly. If the chocolate buttercream (or vanilla) are too thick, just add a tsp of milk and mix in.
Grab a piping bag fitted with a star nozzle and slap the chocolate icing on one side of the bag and the vanilla icing on the other side. Placing the piping bag into a tall pint glass makes this job easier! Pipe onto the top of the cupcakes and finish with a twisted chocolate button.