Heat the oven to 350°F (175°C). Grease and lightly flour two 9-inch round cake pans, or coat them thoroughly with baking spray.

Whisk the flour, sugar, cocoa powder, baking powder, baking soda, and salt together in a large mixing bowl until evenly combined.

Add the eggs, buttermilk, vegetable oil, and vanilla. Beat on medium speed until the batter is smooth and well combined.

Reduce the mixer to low and carefully pour in the boiling water. Once incorporated, beat briefly on high speed for about 1 minute to aerate the batter.

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

Cool the cakes in their pans for about 10 minutes. Turn them out onto wire racks and cool completely before frosting.

Beat the softened butter and cocoa together for the frosting. Add the confectioners’ sugar and milk a little at a time, alternating between them, then beat in the vanilla until smooth and fluffy.

Place one cooled cake layer on a serving plate and spread it with frosting. Add the second layer, then frost the top and sides before slicing and serving.