Preheat oven to 350°F. Line a 12-cup muffin pan with paper liners.

Add 1 cup of spiced apple cider to a small saucepan. Bring to a gentle simmer over medium heat and cook until reduced to ½ cup, about 10–15 minutes. Measure the reduced cider and allow it to cool.

In a large bowl, whisk together the gluten-free flour blend, baking powder, salt, brown sugar, granulated sugar, and cinnamon.

In a separate bowl, whisk together the cooled apple cider reduction, ½ cup melted butter, vanilla extract, milk, and egg.

Add the dry ingredients to the wet ingredients and gently stir until no dry flour remains. Avoid overmixing.

Divide the batter evenly among the 12 muffin cups, filling each approximately ⅔ full.

Bake for 18–20 minutes, or until the tops are set and a toothpick inserted into the center comes out clean or with a few moist crumbs.

While the muffins bake, melt the remaining ¼ cup butter in a small bowl. In a separate shallow bowl, combine the ½ cup granulated sugar and 2 teaspoons cinnamon for the topping.

Allow the muffins to cool in the pan for about 5 minutes, then transfer to a wire rack. While still slightly warm, lightly dip or brush the tops with melted butter, then dip into the cinnamon-sugar mixture. Serve warm or at room temperature.