Preheat oven to 350F (175C). Line muffin pans with paper cupcake liners.

In a large mixing bowl, beat the egg, oil, kefir, red food coloring and vanilla extract. Fold in sugar and salt.

Sift flour and cocoa powder into the same bowl and whisk until combined.

Mix baking soda and vinegar in a small measuring cup and add in the batter when the mixture is still bubbling.

Using an ice cream scoop, fill the liners ⅔ of the way. Bake for 18 minutes or until a toothpick comes out almost clean, just with some crumbs on it. Cool the cupcakes for 5 minutes in the pan and then transfer onto a cooling rack to cool completely.

To make the frosting, using an electric mixer, beat the room temperature butter for 5 minutes until it has become light and airy, lost most of its color and doubled in size. Add the powdered sugar half cup at a time and beat for about a minute in between each addition. Thin out the consistency with milk, adding a teaspoon at a time.

Transfer the frosting into a pastry bag and frost the cupcakes when they are completely cooled.