Preheat your oven to 365F. Use an oven thermometer to test your oven - thermostats on ovens are notoriously inaccurate.
Bloom your cocoa in warm oil. Place the oil in a microwave-safe boil and heat it for one minute in the microwave. This should bring it to about 200F - use a candy thermometer if you're not sure. Stir in the cocoa powder to dissolve, then let it cool (I put it in the fridge while I prep the rest of the batter.)
Prepare 2, 9" round cake pans by greasing and flouring, or by lining with parchment paper. Click here for instructions on making a perfect parchment round.
In a medium bowl (not the mixing bowl), whisk together the flour, baking powder and salt.
In a mixing bowl with whisk attachment, add the eggs one at a time to break up the yolks, then whisk on medium-high to pale yellow and foamy - about two minutes.
Add the sugar and mix to dissolve. Slowly add cooled oil with cocoa and combine. (Hot oil will cook your eggs! Make sure it's cool.) Finally, add the vanilla and combine.
Switch to a paddle attachment. Starting with the flour, add 1/3 of the flour to the bowl and alternate with the milk, ending with flour. Gently mix in the sour cream just until blended.
FOR CAKE: Pour the batter into your prepared pans (I use a scale to divide the batter evenly). Bake for 30 minutes, testing at 25. A toothpick inserted in the center of the cake should come out clean, with just a few, cooked crumbs.
FOR CUPCAKES: Scoop the batter into prepared cups - 2/3 full. The cupcakes won't rise too far, so be generous. Bake for 20 minutes, testing at 18. A toothpick inserted in the center of the centermost cupcake should come out clean, with just a few, cooked crumbs.
Remove and cool in the pan on a rack for 10 minutes. Invert and remove from pan to cool completely.