Preheat the oven to 350 degrees. Line a standard muffin pan with paper liners.

Whisk the flour, cocoa powder and salt in a medium bowl. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar at medium speed for 3-5 minutes, or until light and fluffy. Add the egg on low speed, beating just until incorporated. Scrape down the sides of the bowl as necessary.

Place the buttermilk, red gel paste and vanilla extract in a glass measuring cup and stir well to combine.

Add 1/3 of the flour, followed by one third of the milk, and mix on low. Repeat, adding the remaining flour and milk in 1/3's, scraping down the bowl as needed.

In a small bowl, add the baking soda to the vinegar, which will fizz up. Mix thoroughly and add this reaction to the batter, beating on low speed just until incorporated.

Divide the batter evenly between each muffin cavity and bake for 18-22 minutes (check after 15 minutes), or until a toothpick comes out clean.

Transfer the pan to a wire rack and cool in the pan 10 minutes. Remove from the pan and cool completely.

Place the cream cheese and butter in the bowl of a stand mixer fitted with the paddle attachment and mix on medium high speed until smooth. Add the confectioner's sugar, vanilla extract or vanilla bean paste and a pinch of salt, and mix on low until incorporated, then on medium high for about 3 minutes, until light and creamy.

Pipe or spread the frosting on the cupcakes when they are completely cool.