In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt until they’re evenly distributed.

Place the butter and sugar in a large bowl or the bowl of a stand mixer and use an electric mixer to beat them at full speed for 1-2 minutes. Add the oil and beat for another 1-2 minutes at full speed. 

Add in the eggs, one at a time, beating for at least 30 seconds between each one, until it’s incorporated. 

Add the sour cream and vanilla and mix just until it’s combined.

Add half of the flour mixture, turn the mixer on low speed and stream in the milk. Add the other half of the flour mixture and increase the speed to medium for another 30-60 seconds, just until there are only a few streaks of flour left. 

Use a rubber spatula to scrape the bottom and sides of the bowl and give it one more good mix by hand. 

Distribute the batter evenly among the prepared pans, about 18-19oz in each pan. 

Bake for 30-35 minutes or until the edge of the cake releases from the pan or springs back when you gently press on the tallest part. 

Flip onto a tea towel or a cooling rack and allow to cool to room temperature before frosting. 

Place the sugar and water in a medium saucepan and swirl the pan until the sugar is fully immersed in water. Place the saucepan over medium heat until the sugar dissolves and comes to a gentle simmer. Lower the heat to low and place the lid on the pan and allow the mixture to simmer for a couple minutes. 

Meanwhile, place the eggs in the bowl of a stand mixer, fitted with the whisk attachment, and turn the mixer up to high speed. Let it whip while the sugar is done cooking.

Use a candy thermometer to see when the sugar reaches 240F (116C), then lower the mixer to medium speed and slowly drizzle the hot sugar syrup into the fluffy eggs, while they are whipping. Turn the mixer back up to high speed and let it whip for 5-10 minutes, until the egg mixture is very pale and fluffy and is cool to the touch or no longer warm.

Turn the mixer back down to medium speed and add in the butter, 2 Tbsp of butter at a time, mixing until each chunk is fully combined and the buttercream looks smooth and creamy.

Scrape the sides and bottom of the bowl, add in the cocoa powder, vanilla and salt and turn the mixer up to high speed for 1-2 minutes.

Place the first layer of cake on a flat surface. Frost with 1/4 of the buttercream. Repeat with the second layer and then place the third layer of cake on top.

Make a crumb coat by applying a thin layer around the edge and top of the cake. Refrigerate the cake for 30 minutes.

Use the rest of the buttercream to frost the outside of the cake and use an offset spatula or a spoon to make the swirls.