Preheat the oven to 325 (F) and prepare your donut pan by apply a generous coating each donut cavity with non-stick cooking spray.
In a large bowl, whisk together 1 cup flour, 1/4 cup cocoa powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt.
In a small bowl, whisk together 1/2 cup buttermilk, 1/2 cup brown sugar, 1 egg, 3 tablespoons melted butter, and 1 teaspoon vanilla extract until smooth.
Add the liquid mixture (from step 3) to the flour mixture (from step 2), whisk until blended & smooth.
Spoon batter into your piping (or Ziploc) bag. Cut off the corner and squeeze the batter into the prepared donut pan, filling each cavity about 2/3 full. DO NOT OVERFILL.
Bake for about 13 minutes, or until the donuts spring back when lightly pressed.
Cool donuts in the pan on a wire rack for 3-5 minutes, then carefully turn out onto a wire rack to cool.