Preheat the oven to 160C Fan and line a 12-hole muffin tray with 12 cupcake cases.
Cream the butter in a mixer on the highest speed for 2-3 minutes until lighter in colour and fluffy in texture.
Add the sugar and cream together with the butter, scraping down the sides as neccesary, until well combined.
Whisk in the egg whites one at a time until fully incorporated. Next, add the vanilla and sour cream or yogurt, and whisk again.
In a separate bowl, combine the dry ingredients; flour, ground almonds, salt, Xanthan Gum and baking powder. Whisk until well combined.
Sift in the dry ingredients into the wet mixture, and mix on a low-medium speed until just combined and no pockets of flour remain. Take care not to overmix.
Use an ice cream scoop, to spoon one generous scoop into each of the cupcake cases. This ensures even distribution of the mixture and thus evenly-sized cupcakes.
Bake on the middle shelf for around 20-25 minutes, or until springy to the touch and an inserted toothpick comes out clean with no wet crumbs on it.
Allow the cupcakes to cool for 2-3 minutes before transfering them to a cooling rack to cool completely.
Once cooled, prepare the buttercream by creaming the butter in a mixer on the highest setting for 5-6 minutes, scraping down the sides as neccesary, until the butter becomes much paler in colour and fluffy in texture.
Sift in the icing sugar in small batches and continue to whisk on the highest setting until fully incorportated.
Add the vanilla extract and the double cream and whisk again for 1-2 minutes until light and fluffy.
If using the white food gel colouring add the few drops and whisk until completely incorporated into the buttercream. If using the purple food gel method, add a tiny drop at a time and whisk in well after each one, until the yellow tones of the buttercream fade and the colour is paler and whiter. Be careful here not to add more than a small drop each time, so as not to turn the buttercream purple. (see notes*)
Transfer the buttercream to a piping bag fitted with a large open-star nozzle and pipe a blob in the centre of the cupcake to act as support for the swirl. Starting at the base, pipe the buttercream in a swirl around the cupcake 3 times.
Sprinkle with funfetti rainbow sprinkles to finish.