In a large mixing bowl, combine the melted butter and granulated and dark brown sugars and stir for 1-2 minutes, until the sugar dissolves. Add the eggs, vanilla extract, and buttermilk, and stir.
Sift in the cake flour, Dutch-processed cocoa powder, baking soda, and salt. Gently fold together until no raw flour remains visible-- do NOT overmix! Gently fold in the chocolate chips and morsels.
(Optional): for the best rise, chill the batter for 8-24 hours in the fridge. Otherwise, proceed to the next step.
Generously grease the muffin tin with nonstick cooking spray, then line each cavity with a muffin wrapper. Lightly grease each muffin wrapper, then fill each cavity all the way with batter.
(Optional): sprinkle a dusting of turbinado sugar overtop each muffin, and dot the tops with more chocolate chips just before baking.
Bake on 350℉ for 15-17 minutes, or until a toothpick inserted in the muffins' centers has a few wet crumbs attached. Allow the muffins to cool in the tin for 2-3 minutes, then turn them out onto a wire cooling rack.
