Preheat the oven to 200C/180C Fan/Gas 5. Line an 8" square tin.
Combine the milk and white wine vinegar in a small jug and set aside (essentially creating buttermilk).
In a large bowl, or the bowl of a stand mixer, combine the butter and sugar on medium/high for about 3-5 minutes, until fully "creamed".
Add the egg, milk/vinegar mixture, vegetable oil and red food colouring. Mix again.
Add the flour, cocoa powder, baking powder and baking soda and fold gently until all combined. Take care not to overmix (it should be fairly smooth but a few small lumps is fine!). Pour the cake batter into the lined tin and bake 25-30 minutes until a skewer inserted comes out clean. Let cool on a wire rack.
Once cool, make the frosting. Combine the butter and cream cheese using an electric mixer on high. Add in half the icing sugar, combine, then add the remaining half. At this stage the icing should look smooth and creamy! Feel free to add more icing sugar if you want it thicker.
Spread the icing over the cake, add sprinkles if you like, slice and serve!