Preheat oven to 180C/350F and grease a standard sized 6 hole doughnut pan. (If you have 2, great! Grease them both, but if you have one, like me, bake doughnuts in two batches)
Place the flour, cocoa powder, baking powder and salt into a large bowl and whisk until combined. Leave to one side.
Place caster sugar, vanilla extract and egg into a medium sized bowl and whisk until thick and creamy, about 1 minute.
Add in the melted butter and mix until combined.
Add in the red food colouring, a little at a time, mixing in between until you get the right shade - you want a really bright red!
Add the red mixture to your dry ingredients and mix until you have a thick batter.
Add in the buttermilk and mix until well combined.
Place the bicarbonate of soda and vinegar into a small glass and mix together briefly, mixture will fizz up a little.
Add the vinegar mix into your doughnut batter and mix until well combined. At this stage, if your batter isn't red enough, add a little more colouring!
Place the batter into a large piping bag, with a large plain nozzle attached (I used a 12mm) and pipe batter into each doughnut hole - keep the piping consistent and smooth, you want to fill each hole about 2/3 full.
Alternatively, spoon the batter into the doughnut pan with a small spoon - I would advise piping though as it's much easier!
Place doughnuts in the oven for 8-10 minutes, until well risen and an inserted skewer comes out clean.
Leave in pan for 5 minutes, before transferring to a wire rack to cool completely.
Make sure the doughnut pan is cool before greasing again and piping the remaining 6 doughnuts into it and baking, if you're using one pan.
Once all doughnuts are completely cool, make the frosting: Place all frosting ingredients into a large bowl and beat on med-high speed until light and smooth, about 1 minute.
Using a small angled spatula, or spoon, spread frosting over the top of each doughnut before adding whichever sprinkles/decorations you desire.
Frosted doughnuts will keep in an airtight container, in the fridge for 2 days. Un-frosted doughnuts can be frozen for a month.