Start by making the apple pie mixture. Peel and dice your apples into small cubes. Add the butter to a saucepan, then add your apples, brown sugar, vanilla, apple pie spice, and salt. Bring to a boil, then reduce to a simmer and cook until the apples soften and the liquid reduces to a thick, glossy glaze. Set aside to cool completely.
Make the streusel. Mix the flour, brown sugar, apple pie spice, and salt. Then add the melted butter and mix until big chunks form. Set aside.
Preheat the oven to 375 degrees F. Make the batter. Combine the dry ingredients in a bowl (flour, baking powder, salt, and apple pie spice) and set aside.In a larger bowl, combine the melted butter, milk, eggs, and vanilla. Once mixed, add the sugar and stir to combine. Then, add the dry ingredients and mix until just combined. Spoon about ⅔ of the apple mixture into the batter and fold until just combined.
Add muffin liners sprayed with a bit of non-stick cooking spray to the muffin tin. Use a cookie scoop that measures 4 tablespoons and add it to each tin. This will fill your tin almost to the top, but not all the way — you'll need that extra room for the next step!
Now spoon about a teaspoon or two of the apple mixture onto the top of each muffin. Once done, add about two tablespoons of the streusel, pressing it lightly into the apple mixture. Don't worry if this is a little messy.
Bake in a 375-degree F oven for 25 minutes, or until the tops are golden brown. Let rest in the pan for 10 minutes, then remove to a wire rack to cool completely. This also helps to ensure the tops stay crunchy once you store them.