Preheat your oven to 325° F / 160° C (normal oven) / 140° Fan / Gas 3.
Line a muffin tin with 12 large muffin sized cupcake cases or use two small muffin sized tins.
Using a handheld whisk, whisk together the unsalted butter and sugar until pale and fluffy.
Carry on whisking and add eggs, pinch of salt and the coffee granules dissolved in small amount of hot water.
Add the gluten free flour and the cocoa powder and carry on whisking on a slow speed until all combined.
The mixture might seem a little dry at this point, so add the sunflower oil and the milk. Mix on a slow speed until the mixture is combined.
Using a spatula gently fold the chocolate chips in and mix.
Divide the cupcakes mixture into the cupcake cases (they should be about 2/3 full)
Bake for 25-30 minutes and check with a wooden skewer when done.
Leave in the tin until the cupcakes have cooled down a bit and then remove them from the tin and leave to finish cooling on a wire rack.
To make the buttercream, whisk together the butter, icing sugar and the cocoa powder. Add a pinch of salt and water. Carry on whipping for at least 4 minutes or until the icing is light and fluffy (and goes slightly lighter in colour).
Using a knife spread the buttercream on to the cupcakes (or you can pipe the buttercream if you like.