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Preheat the oven to 350 degrees F. Prepare three 6" or two 8" round cake pans by spraying with baking spray, dust with cocoa powder and shake out the excess. Line the bottoms of the pans with parchment paper and set aside.
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Using a hand mixer or a stand mixer fitted with the paddle attachment, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix on low for 30 seconds or until thoroughly combined.
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Add eggs, buttermilk, sour cream, oil, and vanilla. Beat on medium speed for about two minutes until smooth.
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Reduce speed and slowly add boiling water to the cake batter. Turn mixer to high and beat for about 1 minute until ingredients are incorporated.
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Distribute cake batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a toothpick or cake tester inserted in the center comes out clean.
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Remove from the oven and allow to cool for 15-20 minutes, remove from the pan and cool completely on wire rack.
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Make the frosting: Using a handheld or stand mixer fitted with a paddle attachment, cream the butter on high speed until light and creamy, about 5 minutes.
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Turn mixer to low and slowly add the sifted cocoa powder and sifted powdered sugar 1 cup at a time. Beat until sugar/cocoa until combined.
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Turn mixer to medium speed and add the vanilla and cream 1 tablespoon at a time and beat until smooth.
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Switch to high speed and beat for 1 minute. You may need to add more cream or more powdered sugar depending on consistency desired or if you'd like to increase the frosting's thickness. Taste, and add salt to taste.
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Assemble the cake: Place 1 layer, flat side up, on a plate or cake stand.
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With a knife or offset spatula, spread the top with about 1/2 - 2/3 cup frosting.
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Place the second layer evenly on top, and press down gently so cake layers stick.
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Spread another 1/2 - 2/3 frosting evenly on the top.
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Position the third layer, bottom side up, on top of the frosted layer and add frosting to the top and sides of the cake with a very thin layer of frosting (the crumb coat).
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Refrigerate the cake for about 15 minutes to let the crumb coat set up.
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Ice the sides and top of the cake with the finishing coat.
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Decorate with chocolate chips or as desired.