Pre-heat the oven to 180°C / 160°C fan assisted /gas mark 4 / 350°F and line and grease 2 x 8 inch (20cm) round cake tins (4cm depth).
Beat the butter and caster sugar together in either a stand mixer or a large bowl with an electric beater for about 2 minutes until well mixed, scraping down the sides of the bowl every so often.
Add the eggs one at a time and beat well. Follow with the vanilla extract.
Sift together the cocoa powder, gluten-free flours, bicarbonate of soda and salt into a bowl and set aside for a moment.
Mix together the buttermilk and vinegar in a jug.
Add the flour mixture and buttermilk into the rest of the cake batter alternately. Mixing well after each addition. Add the flour in 3 additions and the buttermilk in 2 additions, starting and ending with the flour.
Stir in the food colouring and mix until the colour is as desired.
Allow the cake batter to rest for 20-30 minutes before giving a final stir and pour into the prepared cake tins.
Bake for 40 minutes or until an inserted cocktail stick comes out clean.
Remove from the oven then rest the sponges for 5 minutes in the tins to allow them to retract from the sides of the tin.
Carefully remove the sponges from the tins to finish cooling on a wire rack.
Using a very sharp knife, cut a very small wafer thin slice of the sponge away (not up to the edges of the cake to avoid it being noticed) and whizz up to cake crumbs in the food processor for decoration.