Preheat oven to 350℉. Spray a 12-count regular muffin pan or a 6 count jumbo muffin pan with nonstick spray or line with cupcake liners.
Whisk gluten-free flour, baking powder, baking soda, and salt in a large bowl. Add melted butter, brown sugar, eggs, sour cream, and vanilla extract. Blend until the batter is thick and creamy. Stir in the chocolate chips and walnuts, if using.
Spoon the batter into the pan, filling them about ⅔ full. Sprinkle each muffin with a little coarse sugar.
Bake until golden brown. A cake tester inserted in the center of a muffin should come out clean, about 18 minutes for standard muffins and about 22 minutes for jumbo muffins.
Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Gluten-Free Flour: This recipe was developed and tested with Bob's Red Mill 1:1 Gluten-Free Flour Blend. Use a gluten-free flour that contains xanthan or guar gum. If yours doesn't whisk a ½ teaspoon into the flour before using.
Chocolate Chips: Use your favorite chocolate chips for these muffins. Mini, regular, and chocolate chunks all work. And a combination is lovely too.