Preheat the oven to 350°F. Prepare 2 round 8" cake pans by greasing and lightly flouring each one.
In a medium mixing bowl, whisk together the flour, baking soda, and salt. Set aside.
In a small bowl, add the cocoa powder and ½ cup of lukewarm water, and whisk to combine. Set aside to cool.
In a large liquid measuring cup, combine the buttermilk, ½ cup water, and vanilla extract. Set aside.
In a large mixing bowl, cream the butter on medium speed with an electric mixer until the butter is smooth and light in color (2-3 minutes).
Add the granulated and brown sugars and cream the mixture on medium speed for 3 to 4 minutes. Stop the mixer and scrape down the sides of the bowl as needed. The mixture will have a sandy appearance.
With the mixer on medium speed, add the eggs and sour cream. Continue to cream, scraping the sides of the bowl as needed, until the mixture is smooth and velvety.
Add in the cocoa mixture and mix just until combined.
Using a wooden spoon or spatula, stir in ¼ of the flour mixture, followed by ⅓ of the buttermilk mixture. Repeat these steps, alternating dry and liquid. Mix just until fully incorporated.
Divide the batter evenly into each pan and bake for 25-27 minutes on the lower oven rack. Check with a toothpick to ensure the cakes are done. Allow the cakes to cool in the pan for 10-15 minutes. Transfer the cakes from the pans to a wire cooling rack to finish cooling.