Preheat your fan oven to 160°C or 325°F. Line your cupcake pan with paper cases.
In a medium bowl sift your flour, cocoa powder, baking powder and salt. Set aside.
In a measuring jug combine buttermilk and red food dye until everything is mixed well and there are no small bits of food colouring in buttermilk.
Using a hand mixer beat the butter for a 1-2 minutes then add the sugar and vanilla extract and beat until combined. Add the egg and beat until everything is pale and fluffy. Stop the mixer and scrape down the sides of the bowl.
Turn the mixer on medium and add flour in three batches, alternating with the wet ingredients. It's important to begin and end with flour mixture. Mix only until combined, you don't want to overmix your batter and risk cupcakes being tough. At the very end, in a small jug combine white vinegar and baking soda and immediately stir in the cupcake batter.
Divide the batter evenly among the paper cases, filling them ⅔ of the way full.
Bake for 18-20 minutes or until toothpick inserted in the middle comes out with a few moist crumbs.
Allow the cupcakes to cool completely on a wire rack before frosting them
Ermine Vanilla Buttercream
In a small saucepan combine flour, sugar, salt and milk. Cook over medium-high heat, stirring constantly with a whisk until it thickens. After it has boiled, cook for another 2 minutes.
Once it's done, transfer it to a bowl, put a cling film over it touching the surface to prevent "skin" forming and let it cool to room temperature. This is your pudding base. You can do this in a fridge, but before using it let it come to room temperature.
Using a stand mixer fitted with the paddle attachment, beat the butter for 5 minutes until it's pale and fluffy. Then add spoonful by spoonful of previously made pudding. Mix until it's all nicely incorporated and with no lumps. Add the vanilla extract and beat for another 30 seconds.
Notes
Please note: I don't feel comfortable converting my recipes to cup measurements because I've never used cups. They are not precise enough, so I definitely advise you to buy a kitchen scale. They are cheap and easy to use.That being said, here is the link for an online converter that may help you convert this recipe to cups, if you prefer it that way. Bear in mind that I don't know if it will be converted the right way and how will the recipe turn out.