Preheat the oven to 325 degrees. Coat a doughnut pan with non-stick spray. In a large bowl, combine the flour, cocoa powder, baking soda, and salt. In a separate, smaller bowl, mix together the buttermilk, brown sugar, eggs, melted butter, and vanilla. Pour the wet ingredients into the dry and use a rubber spatula to gently mix the ingredients together. Lastly, mix in the chocolate chips.