Make the 2 cake layers: Preheat oven to 325F (150C) and spray 2 round cake pans with cooking spray. Line the bottoms of each pan with a round of parchment paper, avoiding the sides.
Stir the oil and cocoa mixture into the almond flour mixture. Now add in the remaining wet ingredients: sour cream, beaten eggs, and vanilla. Stir well until fully incorporated. Be sure to scrape the sides and bottom of the bowl.
Pour the batter evenly into the two cake pans. You may want to weight each pan on a kitchen scale to be sure the same amount of batter is in each one. Bake in the center of the preheated oven until a toothpick comes out with a few crumbs.
Make the buttercream: In the bowl of a stand mixer fitted with the paddle attachment, or using an electric mixer, beat the butter on medium speed until creamy, about 2 minutes.
Add in the powdered sweetener, sifted cocoa powder, salt, sour cream, and vanilla. Beat on low speed for 30 seconds, then increase to high speed and beat for about 1 minute more.
If frosting seems too thick, add in the heavy cream and beat for a 10-20 seconds longer. The heavy cream needs to be room temperature. Cold cream will affect the texture of your icing.
Assemble the layer cake: Remove one of the cakes from the pan and place on a cake plate with the bottom of the cake facing up. Remove the sheet of parchment paper from the bottom and discard.
Cover with a ½ inch layer of frosting. Repeat with the second layer, placing it evenly on top of the frosted cake. Spread remaining frosting all over the tops and sides.