Preheat your oven to 350°F (175°C) and line a cupcake tin with liners.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the milk, vegetable oil, vanilla extract, and egg. Mix until smooth.
Fill the cupcake liners about ⅔ full with the batter.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean. Let cool completely.
For the frosting, beat the butter until creamy. Gradually add powdered sugar, peanut butter, heavy cream, and vanilla extract. Beat until fluffy.
Frost the cooled cupcakes and top with chopped Snickers bars.