To top these tender muffins with streusel, just swap the coarse sugar for a crumb topping. Stir together ½ cup all-purpose flour, ½ cup granulated sugar or brown sugar, and a pinch of salt. Add ¼ cup unsalted butter diced into cubes and work the butter into the flour mixture with your fingers until a crumbly mixture has formed. Sprinkle the streusel over the tops of the muffins before baking.