Crumble: Whisk together flour, brown sugar, granulated sugar, cinnamon, and salt. Add the diced butter. Use your fingers to work the butter into the flour until it forms a crumbly mixture. You can squeeze a handful and break it apart to get bigger chunks. Refrigerate until ready to use.
Apple filling: In a medium pot add apples, butter, brown sugar, cornstarch, cinnamon, and nutmeg over medium-low heat. Stir until butter is melted and sugar is dissolved.
Bring to a simmer, stirring occasionally, until apples are tender. Set aside.
Muffins: Preheat oven to 425°F. Line a muffin tin.
Whisk together flour, baking powder, cinnamon, and salt. Set aside.
In a large bowl beat the butter, brown sugar, and granulated sugar until light and fluffy.
Mix in eggs and vanilla.
Add the dry ingredients to the creamed mixture alternately with the milk. Mix until just combined.
Toss apples in 1 tablespoon of flour and fold into batter.
Scoop batter evenly into each cupcake liner. Top each muffin with a spoonful of apple filling, then sprinkle with crumble.
Bake for 5 minutes at 425°F, then reduce oven temperature to 350°F. For regular sized muffins bake an additional 15–18 minutes. For jumbo muffins bake an additional 25-30 minutes.
Optional: Drizzle caramel sauce on the muffins, if desired.
