First, brown the butter: cook the ½ cup unsalted butter over medium-low heat until it melts and starts to brown a bit (about 5 minutes). Stir and watch it carefully so that it doesn't burn. Set aside to cool.
In a large bowl, whisk the ⅓ cup whole milk and 2 large eggs to combine, then whisk in the butter. Stir in the 1 ¾ cups gluten free flour, ¾ cup granulated sugar, 2 teaspoons baking powder, ½ teaspoon fine sea salt, and 1 teaspoon ground spices of choice. Fold in the 1 cup apples.
Divide the batter equally between the muffin cups in the prepared pan and preheat the oven to 375℉. Rest the batter while the oven heats, or for up to 20 minutes.
Place the muffins in the oven and bake for 25-30 minutes, until very lightly browned and a tester inserted comes out clean. Cool in the pan set on a wire rack for 10 minutes, then turn out the muffins to cool completely.
Notes
Let the batter rest while the oven preheats - Gluten-free flours benefit from a rest to let them absorb liquids in the batter, which makes for a better structured and tender final product.
Use your favorite gluten free flour - I love using my homemade gf flour blend in these muffins, but any brand of store-bought gluten free all-purpose flour will work just fine.
Make them dairy-free - To make dairy-free apple muffins, substitute your favorite non-dairy milk for the milk and use your favorite non-dairy butter for the butter. You could also use a neutral oil (½ cup) for the butter, and omit the browning step.
Don't brown the butter - I get it - sometimes we're short on time. If you don't have time to brown the butter, no worries, these muffins will still taste delicious.
Other sugars - For a deeper, more caramelly note of sweetness, use light or dark brown sugar instead of the granulated sugar. Or, substitute maple syrup or coconut sugar for the sugar.