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

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

Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Mix with an electric mixer on medium speed until just combined.

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

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

Bake for 18-20 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean.

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

In a large bowl, beat the softened butter with an electric mixer until light and fluffy.

Gradually add the powdered sugar and cocoa powder, beating on low speed until combined, then increase speed to medium and beat until smooth.

Add the milk and vanilla extract. Beat on medium-high speed until the frosting is light, fluffy, and smooth.

Once the cupcakes are completely cool, frost them using a piping bag or an offset spatula.