Combine the melted butter and brown sugar in a small bowl. Stir in the gluten-free flour, pecans, cinnamon, and salt. Set aside while you prepare the muffin batter.
Make the Muffins
Heat the oven to 350°F. Line a standard 12-cup muffin pan with paper liners.
Whisk together the pumpkin purée, eggs, milk, oil, and vanilla extract in a medium bowl. Set aside.
In a large mixing bowl, whisk together the gluten-free flour, granulated sugar, brown sugar, pumpkin pie spice, cinnamon, baking powder, baking soda, and salt.
Add the softened butter to the dry ingredients. Mix on low-medium speed until the butter is evenly distributed and the mixture resembles damp sand. There should be no visible pieces of butter.
Add the pumpkin mixture. Mix on low speed until combined, then increase to medium and beat briefly until the batter is thick and smooth. Scrape down the bowl as needed. Avoid prolonged mixing.
Divide the batter evenly among the 12 muffin cups, using a ¼-cup (2-ounce) scoop.
Top each muffin with approximately 1 tablespoon of streusel. For large, crunchy clusters, squeeze portions of the streusel together, then break them into uneven pieces over the batter. Gently press the streusel onto the batter so it sticks.
Bake for 22-25 minutes, or until the tops spring back when lightly pressed and a toothpick inserted into the center comes out clean or with a few moist crumbs. The internal temperature should read 205-210°F.
Cool the muffins in the pan for 10 minutes, then transfer them to a wire rack to cool completely.
Make the Maple Glaze
Stir together the powdered sugar, maple syrup, vanilla extract, and salt in a small bowl.
Add ½ teaspoon of milk, then continue adding milk a little at a time until the glaze is smooth and thick but easy to drizzle.
Drizzle the glaze over the cooled muffins and serve.
Notes
Gluten-free flour: This recipe was tested with Bob’s Red Mill 1:1 Gluten-Free Flour. For best results, use a similar measure-for-measure blend that contains xanthan gum and weigh the flour.
Pumpkin: Use 100% pure pumpkin purée, not pumpkin pie filling.
Storage: These muffins are best the day they are baked but can be stored in an airtight container at room temperature for 1 to 2 days. For longer storage, freeze the muffins without the glaze for up to 2 months. Thaw uncovered at room temperature, then add the glaze before serving.