Preheat oven to 350°F (180°C). Line a cupcake/muffin tin with 4 liners and set aside.

In a medium bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.

In a small bowl, whisk together the milk, vinegar, oil, and vanilla. Pour the wet ingredients into the center of your flour mixture and stir until just incorporated. Careful not to overmix!

Divide the batter among the 4 large cupcake liners and bake for 18 minutes or until a toothpick inserted in the center comes out clean. Allow to cool.

To make the frosting, beat softened butter until smooth and creamy. Add powdered sugar, peanut butter, and milk and beat until combined. Add more powdered sugar if frosting is too thin or more milk if mixture is too thick. Add salt if frosting is too sweet (1/4 teaspoon). Frost cooled cupcakes. There may be leftover frosting depending how much you use on each cupcake.