Generously mist three 8-inch diameter by 2-inch high cake pans with non-stick spray, and line with circles cut from parchment paper.
Place the sugar, flours, cocoa, baking powder, baking soda, and salt in a large mixing bowl, and whisk to combine.
Cut the butter into smaller pieces and add it to the dry ingredients.
Mix on low speed until the mixture resembles damp sand (30 seconds to a minute).
Add the eggs, one at a time, scraping the bottom and sides of the bowl with a silicone spatula after each addition.
When all of the eggs are fully incorporated, add the Greek yogurt (or sour cream) and vanilla, and beat on medium speed for about 60 to 90 seconds, to aerate the batter and strengthen the cake's structure.
Divide the batter evenly between the three prepared pans, and bake for 30-40 minutes, or until a toothpick inserted in the thickest part of the cake comes out clean or with one or two moist crumbs.
Cool completely, then soak with cocoa simple syrup (optional), and fill and frost.
TO MAKE THE COCOA SIMPLE SYRUP (OPTIONAL):
Place the sugar and cocoa powder in a small pot and whisk to combine.
Stir in the water and place over medium heat.
Cook, stirring occasionally, until all the sugar has dissolved.
Place one cake layer on a serving platter. Use a pastry brush to soak the cake with the simple syrup before topping with filling. Repeat as you add each layer.