First, brown the butter: cook the 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 milk and eggs to combine, then whisk in the butter. Stir in the flour, baking powder, salt, and spices. Fold in the 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.