Preheat oven to 350 degrees. Line a cupcake pan with paper liners.
Combine the flour, sugar, cocoa powder, baking powder, baking soda, cinnamon, and salt together in a bowl and whisk together.
Combine the egg, milk-ACV mixture, oil, and vanilla in another bowl and whisk together.
Add the wet ingredients to the dry ingredients and mix using a stand mixer (or in a bowl with a hand mixer) until combined.
Add the hot water to the batter and mix again, scraping the sides of the bowl as necessary, until combined. Batter will be thin.
Fill cupcake liners only 1/2 full, then bake for 18-21 minutes, until the tops bounce back when you touch them with a finger or a toothpick comes out with only a few moist crumbs when used to test a cupcake.
Cool completely before frosting.