• Preheat the oven to 180'C / Fan 160'C / Gas Mark 4. Fill a 12-hole baking tin with cupcake cases and set aside while you make the cake batter.
• Add the butter, sugar and vanilla extract to a large mixing bowl and beat with an electric whisk until pale and creamy. (TIP: If the butter is hard, zap it in the microwave for 10-15 seconds before adding it to the mixing bowl to make it easier!)
• Crack the eggs into the mixing bowl and beat again for 30 seconds to combine. Scrape down the sides of the bowl using a spatula.
• Sift in the self-raising gluten free flour, cocoa powder and xantham gum, then add the milk and beat together with the electric whisk until everything is fully combined. Pour in the white chocolate chips and fold them into the mixture so they're evenly distributed. You want a fairly thick batter which looks light and fluffy.
• Spoon the mixture evenly between the cupcake cases and then place in the centre of an oven and bake for 20-22 minutes. The cupcakes are done when they are well-risen and a skewer inserted in the middle comes out clean.
• Once cooked, remove from the oven and place on a cooling rack. Cool in the tray for 10 minutes before removing the cakes from the tray to cool completely.

To make the frosting:

• Break the dark and milk chocolate into chunks and melt either in a bowl over a pan of hot water, or by microwaving for 20-second intervals. Stir to mix together and set to one side to cool slightly.
• Add the butter, icing sugar, melted chocolate (cooled) and milk to a large mixing bowl and beat with an electric whisk on a low speed until it becomes a smooth, fluffy buttercream, almost like a chocolate mousse. (TIP: To avoid an icing-sugar dust cloud I tend to 'mash' the mixture with a fork until it starts to come together, then use the electric whisk).
• Spoon the mixture into a piping bag (see notes) and, once the cupcakes are completely cool, decorate them with a swirl of buttercream. I used a 2D piping tip. Finish with a sprinkle of white chocolate chips.