Whisk cocoa powder and sugar together in a bowl until well combined. Add flour, brown sugar, baking soda, baking powder, and salt; continue whisking until evenly mixed. Make a well in the center of the bowl.

Whisk buttermilk, eggs, vegetable oil, and vanilla extract together in a separate bowl. Pour into center of the well of the cocoa mixture; mix until just blended.

Heat a lightly oiled griddle over medium heat. Drop 1/4 cup batter onto the griddle and cook until bubbles form and the edges are dry, 3 to 4 minutes. Flip and cook until browned on the other side, 2 to 3 minutes. Repeat with remaining batter.