Preheat oven to 400°F. Spray your muffin tin with cooking spray if it isn’t non-stick.
Roll out the pie crust and use a round cookie cutter to cut out circles.
Press each dough circle into the muffin tin, crimping the edges to fit.
In a bowl, combine diced apples, brown sugar, cinnamon, and flour. Stir until apples are well coated.
Spoon 2–3 tablespoons of filling into each tart shell, leaving a little space at the top.
In another bowl, add the flour, brown sugar, and cinnamon for the crumble topping.
Melt the butter in the microwave and pour it into the crumble mixture. Pro tip: Cut butter into small chunks before microwaving and heat in 30-second intervals.
Stir the crumble ingredients until a paste-like mixture forms.
Spoon a thin layer of crumble on top of each tart.
Bake at 400°F for about 20 minutes, or until the filling is bubbly and the apples are soft. Let the tarts cool completely before removing from the tin. Top with ice cream or whipped cream if desired.