Preheat the oven to 350°F (170C/155C Fan) and line a cupcake tray with cases.
In a small bowl, make a paste with the cocoa powder and vinegar. Set this aside.
In a mixing bowl, beat together the sugar and butter until creamy, light and fluffy. Add the egg and beat to combine.
Add the cocoa powder paste and beat until combined.
Sift the cornflour, baking soda, salt and half of the flour into the butter mixture and beat to combine.
Add the soured milk, beating until just mixed and then sift in the remaining half of the flour, stirring to combine.
Finally add the vanilla and food colour and lightly stir until fully incorporated and a light red shade.
Divide the batter evenly amongst the cases and bake for 20-25 minutes, until a toothpick inserted in the centre of a central cupcake on the tray comes out clean. Allow to cool on a wire rack.