Preheat your oven to 350 degrees Fahrenheit. Grease and flour two 9-inch round pans or line them with parchment paper.

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

In another bowl, mix the eggs, milk, oil, and vanilla extract until smooth. Pour the wet mixture into the dry ingredients and stir until just combined.

Slowly add the boiling water to the batter while stirring. The batter will be thin, which is expected.

Divide the batter evenly between the prepared pans. Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.

Let the cakes cool in the pans for about 10 minutes, then transfer them to a wire rack to cool completely.

To make the frosting, beat the butter with a mixer until fluffy. Add the cocoa powder and mix until combined.

Add powdered sugar, milk, vanilla extract, and a pinch of salt. Beat until the frosting is smooth and spreadable, adjusting milk as needed.

Place one cake layer on a serving plate and spread frosting on top. Add the second cake layer and frost the top and sides of the cake.