Preheat oven to 350°F (180°C). Grease and flour two 9-inch (23 cm) round cake pans (or you can use three 8" (20 cm) round cake pans). Set aside.

In a mixing bowl, whisk together cake flour, baking powder, and salt. Set aside.

In a second mixing bowl with an electric mixer on medium speed, cream butter, oil and sugar together until smooth and combined about 2 to 3 minutes.

Add the eggs one at a time, beating well after each addition. Scrape down the sides and bottom of the bowl after each addition. Add vanilla extract. Beat to combine.

Alternate adding the flour mixture with the buttermilk, beginning and ending with the flour mixture. Do not overmix. Stop mixing once the last of the flour has been mixed in.

Divide batter between your cake pans, smooth the tops. Bake for about 30-35 minutes. The cake is done when it's golden brown on top and toothpick inserted into the center comes out clean or with a few moist crumbs. Remove cake from the oven and allow it to cool for 5 minutes, then turn it out onto a rack to cool completely.

Combine the butter and confectioners sugar in the bowl of your stand mixer. Mix on low speed at first, then increase the speed to medium until combined.

Add in your heavy cream and vanilla extract and mix on low speed until combined, about 1 to 2 minutes.

With the mixer off, add cocoa powder. Mix on low speed until combined. Once the cocoa powder has been mixed in, increased to medium-high speed and mix until light and fluffy, about 5 to 7 minutes.

Once the cake has cooled, frost the cake with chocolate buttercream frosting. Place bottom layer onto cake stand or serving platter. Using an offset spatula, spread a layer of frosting on to the bottom cake layer. Place second layer on top. Frost top and sides with more frosting until smooth.