Preheat the oven to 350 degrees F.

Whisk the cocoa powder and 1 1/2 cups boiling water in a medium bowl until smooth; set aside.

Whisk the flour, sugar, baking powder, baking soda and salt in a large bowl until combined.

Add the eggs, vegetable oil, sour cream and vanilla and beat with a mixer on medium speed until smooth, about 1 minute.

Divide the batter between the prepared pans and tap the pans against the counter to help the batter settle.

Transfer to racks and let cool 10 minutes, then run a knife around the edge of the pans and turn the cakes out onto the racks to cool completely.