Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.

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

In a separate bowl, whisk together eggs, vegetable oil, buttermilk, and vanilla extract.

Add the wet ingredients to the dry ingredients and mix until just combined. Do not overmix.

Carefully stir in the hot water. The batter will be thin.

Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.

Bake for 20-24 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cupcakes cool in the muffin tin for 10 minutes before transferring them to a wire rack to cool completely.

In a large bowl, beat the softened butter until creamy.

Add the cocoa powder and beat until combined.

Gradually add the powdered sugar, alternating with the milk, and beat until smooth and creamy. Start and end with powdered sugar.

Stir in the vanilla extract.

Frost the cooled cupcakes as desired.