Preheat oven to 375F and line 2 12-count muffin tin with liners.
In a large bowl, sift together the flour, pumpkin pie spice, baking soda, and salt. Set aside.
Pour brown sugar into a separate bowl. Pour melted butter over sugar and whisk until the sugar is dissolved. Add the egg, buttermilk, and vanilla extract and combine.
Pour the wet ingredients into the dry ingredients and combine. Fold in the apples. Scoop the batter into the muffin wells, just about full. Place muffins in oven and bake for 20 minutes.
While the muffins are baking, begin to make the streusel topping. Combine butter (slightly chilled), sugar, cinnamon, flour, and salt in a bowl and cut with a pastry blender until the butter is completely combined. The mixture will be crumbly but soft and will become a light brown color. Set aside.
Remove muffins from oven after 20 minutes and sprinkle streusel topping over top. Place back in oven and bake for an additional 5 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean. Let cool in pan for about 5 minutes and then allow the muffins to finish cooling on a wire rack.
*The larger your apple pieces, the less they’ll bake into the muffin. I cut my apple pieces into ½ in cubes so I could have bits of apple in my muffins.