Preheat the oven to 175 C. Line 12 muffin moulds with cupcake liners or grease the moulds.
Sift the flour, cocoa and salt in a bowl and set aside.
In a mixing bowl, beat the sugar and oil with a hand mixer on medium speed, until combined. Add the egg and vanilla and beat to combine. Stir in the food colour.
Stir in the flour mixture and buttermilk alternately, starting and ending with the flour to stabilise the batter. If needed, run the hand mixer through the batter to break up any lumps.
In a small bowl, stir together the baking soda and vinegar. It will foam and smell like a chemistry lab in school! Add this to the batter and beat on low speed for 10 seconds.
Divide the batter among the muffin moulds, filling them about 3/4th of the way. Bake for 20 minutes, rotating the tray halfway through, until a toothpick poked in the center comes out clean.
Allow the cupcakes to cool completely at room temperature.
To make the frosting, combine all the ingredients in a mixing bowl and beat for about 2 minutes with a hand mixer, until creamy and light. If it seems too soft, place the bowl in the fridge for 15 minutes, then spread all over the cake. Feel free to add more sugar if you prefer sweeter frostings, or even double the recipe for a thick layer of frosting on each cupcake.
Pipe or spread the frosting onto each cupcake and enjoy! Store in the fridge in an airtight tin, for about 4 to 5 days. Happy baking!