Preheat the oven to 350°F. Spray a 9×13 baking dish with baking spray (Baker’s Joy or generic version). Set it aside.
In a large mixing bowl, sift together the flour, cocoa powder, baking powder, and baking soda.
Whisk in the salt and granulated sugar.
Whisk in the buttermilk, vanilla extract and oil.
Whisk in the eggs, 1 at a time, until well incorporated.
Carefully whisk in the boiling water until all the ingredients are combined. The batter will be thin, but it is supposed to be.
Pour into the prepared baking dish and bake for 30 to 35 minutes, or until a toothpick inserted comes away clean. Allow the cake to cool completely.
Chocolate Frosting
Using either a stand mixer or medium-sized mixing bowl and a handheld mixer on medium-high speed, beat the butter and vanilla for 30 seconds.
Add the cocoa powder and continue mixing until well incorporated.
Add the milk and continue mixing until combined.
Lower the mixer speed to low and add the powdered sugar. Once the powdered sugar is no longer “dusty”, increase the mixer speed to medium and beat for 1 to 1½ minutes until fluffy.
Using either a silicone spatula or an offset spatula, evenly frost the top of the chocolate cake. Slice twelve 3-inch by 3½-inch slices.
Notes
Make sure that your eggs are at room temperature as they will combine better with the batter.
The hot water is key to helping the cocoa powder incorporate well into the batter and won’t leave you with pockets of dry powder.
Allowing the cake to cool before frosting is important as the frosting won’t spread well and will just melt if the cake is still warm.