Preheat the oven to 375°F. Grease a 6-cup muffin pan.
Stir together the dry ingredients and set aside.
Whisk together the melted butter or oil, egg, and milk.
Using an electric mixer, slowly beat the milk mixture into the dry ingredients. Scrape the bottom and sides of the bowl and continue beating at medium speed until the batter thickens, about 30 seconds.
Stir in the berries.
Fill the muffin cups almost full. Sprinkle with cinnamon-sugar.
Bake the muffins for 20 to 25 minutes, until a cake tester inserted into the middle of a muffin comes out clean.
Remove from the oven, and after 5 minutes transfer to a rack to cool.
*Make your own blend
Many of King Arthur's gluten-free recipes use their King Arthur Gluten-Free Multi-Purpose Flour, which includes ingredients that reduce the grittiness sometimes found in gluten-free baked goods. Their flour also increases the shelf life of your treats, keeping them fresh longer.
The following make-at-home blend, featuring stabilized brown rice flour, works pretty well when substituted; and it tastes better than a blend using regular brown rice flour.
Whisk together 6 cups (32 ounces) King Arthur stabilized brown rice flour; 2 cups (10 3/4 ounces) potato starch; and 1 cup (4 ounces) tapioca flour or tapioca starch. Store airtight at room temperature. Note: You can substitute white rice flour for the brown rice flour if you like; it'll make your baked goods grittier (unless you manage to find a finely ground version).