1. Preheat the oven to 400 degrees F (200 degrees C). Line a 12-cup (2 1/2-inch) muffin tin with paper liners.

2. Whisk 1 1/2 cups gluten-free flour, buckwheat flour, baking powder, and salt together in a large bowl. Make a well in the center and set aside.

3. Whisk eggs, milk, orange juice, 1/3 cup brown sugar, oil, and 1 teaspoon orange zest in a medium bowl until combined. Add egg mixture and 1/2 cup chocolate chips to the flour mixture; stir until just combined. Divide batter evenly among the prepared muffin cups, filling each with about 1/4 cup.

4. Whisk remaining 1/2 cup gluten-free flour and remaining 1/3 cup brown sugar together in a medium bowl. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Stir in remaining 1/4 cup chocolate chips and remaining 1/2 teaspoon orange zest until just combined. Crumble about 2 tablespoons streusel mixture over each muffin.

5. Bake in the preheated oven until a wooden toothpick inserted into the centers comes out clean, 18 to 20 minutes. Let cool in the pan for 5 minutes. Remove from pan, place on a wire rack. Let cool completely, about 1 hour.

Cook's Note:

If you wanted to add a simple glaze to these, whisk together 3/4 cup confectioners' sugar and 1 tablespoon whole milk in a small bowl until smooth; drizzle over cooled muffins.