Preheat the oven to 350°F (180°C) and line a cupcake tray with 12 cupcake liners. Set aside.
Sift the all purpose flour into a mixing bowl and add the sugar, baking soda, salt and cocoa powder and mix together.
Prepare homemade buttermilk by adding 1 Tbsp of lemon juice to a measuring jug and then adding room temperature milk up to the 1 cup (240ml) line. Let it sit for a few minutes to curdle.
Add the homemade buttermilk, vanilla extract, egg, canola oil, white vinegar and red gel food coloring and mix into a batter (don't overmix). Use a hand whisk briefly if needed to remove any lumps.
Divide the batter evenly between the 12 cupcake liners.
Bake for 20 minutes or until a toothpick inserted into the center of one of the cupcakes comes out clean.
Remove from the tray and leave them to cool on a wire cooling rack.
Allow to cool completely before frosting.