Preheat the oven to 350 degrees and fill a large muffin tin with cupcake liners.
In a large mixing bowl, use an electric hand mixer on medium speed to whisk together the butter and sugar. Mix the ingredients continuously for about 2 minutes to ensure the mixture is mixed and creamy.
Add 2/3 cup of the milk and the eggs. Mix again on medium speed for about 45 seconds, or until the eggs are fully incorporated into the mixture. Your batter should be quite runny at this point.
Gently mix in the baking powder, cocoa powder, and flour on low speed until just combined, meaning no pockets of dry ingredients remain in the cupcake batter.
Divide the batter among the 12 cupcake liners and bake for 20 minutes.
While the cupcakes are baking, prepare your peanut butter frosting. In a medium-sized mixing bowl, combine the peanut butter, powdered sugar, and remaining milk. Whisk on medium-high speed with an electric hand mixer until the frosting is smooth.
Allow the cupcakes to cool for at least 15 minutes before frosting them. Cut the Snickers into small pieces and set aside. Frost the cupcakes as desired, then top with diced Snickers.