01 - Preheat oven to 350°F. Grease a mini muffin tin or line with mini muffin papers.
02 - In a small bowl, combine the diced apple, brown sugar, and 1/2 teaspoon cinnamon. Set aside.
03 - Whisk flour, granulated sugar, baking powder, 1 teaspoon cinnamon, and salt in a large mixing bowl.
04 - In a separate bowl, whisk together egg, milk, melted butter, and vanilla extract until smooth.
05 - Pour wet ingredients into dry mixture and stir gently until just combined. Do not overmix.
06 - Fold apple mixture, mini marshmallows, chocolate chips, caramel pieces, and graham cracker crumbs into the batter.
07 - Use a spoon to evenly distribute the batter into the mini muffin tin, filling each well about three-quarters full.
08 - Bake at 350°F for 14–16 minutes, or until golden and a toothpick inserted into the center comes out clean.
09 - Remove from oven and allow muffins to cool in the tin for 5 minutes.
10 - While muffins are still warm, brush tops with melted butter and sprinkle with graham cracker crumbs. Add mini chocolate chips and mini marshmallows if desired.
11 - Serve warm or at room temperature.