Preheat oven to 350°F.Grease and flour (3) 8” round cake pans, then line bottom with round parchment paper. Set aside.
Cream the butter and sugars in a large mixing bowl. Add in the eggs and vanilla and mix until well blended.
In a separate large mixing bowl, combine flour, baking soda, baking powder, and salt.
Mix the cocoa powder and boiling water in a medium bowl until well combined and no lumps remain.
Alternating between the two, slowly add dry ingredients and chocolate mixture to the bowl with the butter and sugar. Mix on low speed to incorporate ingredients.
Divide the cake batter evenly between the three cake pans and place in the pre-heated oven. Bake 22-28 minutes, or until toothpick inserted in the center of the cake comes out clean.
Remove from oven and set aside to cool for approximately 10 minutes before transferring the cakes from the pans to a wire rack to cool completely, approximately 20 more minutes. (Cakes should no longer be warm to the touch before frosting).
To assemble the cake, place the first layer top-side down on a cake plate. Add 1/3 of the frosting to the top of the cake layer and spread it out evenly. Take the second layer and level off the rounded top with a knife or cake leveler. Place the second layer on top of the first layer, top-side down. Add frosting to this layer and spread out evenly. Repeat this with the 3rd layer, and spread the remaining frosting out on the top layer.
Mix the butter and cocoa powder in a large bowl until well blended
Add the powdered sugar and mix on low-speed until all of the sugar is combined.
Stir in milk and vanilla extract to reach smooth consistency.