Step 1

Preheat the oven to 350 degrees F with the oven rack in the middle position.

Step 2

Grease two 9-inch cake pans with butter, then line the bottoms with parchment paper.

Step 3

Sift the flour, 3/4 cup of the cocoa powder, the baking powder, and the baking soda into a large bowl.

Step 4

Add the white sugar and the salt to the flour mixture, whisking briefly to combine.

Step 5

Add the eggs, 1 cup of the milk, the oil, and 2 teaspoons of vanilla extract to the dry mixture and whisk well until lump free, about 30 seconds.

Step 6

Add the boiling water and whisk to incorporate. The batter should be very thin.

Step 7

Pour the batter into the prepared cake pans.

Step 8

Bake until a cake tester inserted into the center comes out clean, about 35 minutes.

Step 9

Cool the cakes for 10 minutes before turning them out of the pans onto wire racks.

Step 10

While the cakes cool, place the butter in the bowl of a stand mixer with a paddle attachment.

Step 11

Beat the butter on medium-high speed until pale, about 3 minutes.

Step 12

Add 1/2 of the icing sugar and 1/2 of the remaining cocoa powder and beat, starting slow and gradually increasing the speed to medium-high, about 3 minutes in total.

Step 13

Add the remaining icing sugar and the remaining cocoa powder to the butter mixture, beating again for 3 minutes.

Step 14

Add the remaining milk and the remaining vanilla extract to the butter mixture, beating again on medium-high speed until incorporated, about 1 minute.

Step 15

When the cakes have cooled completely, frost them to your taste with the chocolate buttercream.

Step 16

Serve.