Preheat your oven to 350°Fahrenheit and line three 8-inch round cake pans with parchment paper. Spray each one with nonstick spray then line the bottoms of the pans. Set aside.
In a large bowl, whisk together the flour, sugar, cocoa powder, salt, baking powder and baking soda together.
Add in the eggs, vanilla extract, milk, oil and hot water into the dry ingredients and mix until smooth. Stir in the mini chocolate chips.
Divide the chocolate cake batter between the 3 cake pans and bake for 20-26 minutes. You’ll know they’re done when a toothpick inserted in the middle of the cake comes out clean with no crumbs.
Let the cakes cool in the pan for about 20 minutes, then remove and cool completely on a wire cooling rack.
Once the cakes are cool you can make the buttercream. In the bowl of a stand mixer or in a large bowl with a hand mixer, beat the butter (and shortening, if using) together until smooth. Add the cocoa powder and beat together.
Add the vanilla extract and the icing sugar, ¼ cup at a time until it’s the consistency you like. If you think the buttercream gets too thick, you can add 1 tablespoon of milk at a time until it’s the consistency you want. If the chocolate frosting gets too thin, add ¼ cup of confectioners’ sugar at a time.
Assemble the cake! Place the first cake layer on a cake plate and place some of the chocolate frosting on top and smooth it over. Repeat this once more, then place the final cake on top and frost the sides and top of the cake. Decorate as you wish!