1. Heat oven to 350 degrees . Coat the cups of 2 mini muffin pans with nonstick cooking spray (22 per batch).
2. In a medium bowl, whisk cake flour, cocoa powder, baking powder, baking soda and salt. In a large bowl, beat softened butter and brown sugar until smooth. Add egg and beat until creamy, 2 minutes. On low speed, beat in half the flour mixture, then the buttermilk, followed by remaining flour mixture. Stir in vanilla. Divide batter into prepared pans, about 1 tbsp batter per cup.
3. Bake at 350 degrees for 12 to 14 minutes. Cool 10 minutes in pans on wire racks, then remove directly to racks to cool. Repeat with a second batch of 22.Topping
4. While duffins cool, melt butter in microwave, 30 seconds. Combine granulated sugar and cocoa in a small bowl. Dip duffin tops in melted butter, followed by cocoa sugar. Serve slightly warm.