Preheat your oven to 350°F (175°C). Line a mini muffin tin with mini cupcake liners.

In a medium-sized bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.

In a large bowl or a stand mixer, whisk together the granulated sugar, brown sugar, and vegetable oil until well combined.

Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix.

Spoon the batter into the prepared mini muffin pan, filling each liner about 2/3 full.

Bake in the preheated oven for 10-12 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Your baking time may vary a little depending on your oven.

Remove the cupcakes from the oven and allow them to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.

While the cupcakes are cooling, prepare the buttercream frosting. In a large mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar, one cup at a time, beating well after each addition. Add the vanilla extract and enough heavy cream or milk to achieve your desired consistency. Beat until light and fluffy.

Once the cupcakes are completely cooled, pipe or spread the buttercream frosting onto the cupcakes using a piping bag or offset spatula.

Decorate the frosted cupcakes with sprinkles, chocolate shavings, or any other toppings of your choice.

Serve and enjoy these delightful mini chocolate cupcakes with buttercream frosting! Enjoy and happy baking!