Preheat the Oven:
Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.

Make the Cupcake Batter:
In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. In a separate large bowl, whisk together the granulated sugar, brown sugar, eggs, vanilla extract, milk, and vegetable oil. Add the dry ingredients to the wet ingredients in batches, mixing until combined. Stir in the sour cream for extra moisture. Finally, carefully pour in the boiling water and mix until smooth (the batter will be thin, which is normal).

Bake the Cupcakes:
Divide the batter evenly among the cupcake liners, filling each about 2/3 of the way full. Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

Make the Chocolate Frosting:
While the cupcakes are cooling, prepare the frosting. In a medium bowl, beat together the softened butter and cocoa powder until smooth. Gradually add the powdered sugar, vanilla extract, and salt. Add the milk a little at a time until you reach a smooth and spreadable consistency. If the frosting is too thick, add more milk. If it’s too thin, add more powdered sugar.

Frost the Cupcakes:
Once the cupcakes are completely cool, frost them with the chocolate frosting using a spatula or a piping bag. For an extra touch, top with chocolate shavings, sprinkles, or a small piece of chocolate.

Serve and Enjoy:
Serve the cupcakes and enjoy the rich, fudgy goodness!