In a medium mixing bowl, combine the flour, sugar, cinnamon, and salt.

Stir in the melted butter, mixing just until combined. Pop in the fridge while you prepare the rest of the recipe. The mixture will absorb the butter as it sits and become more crumbly by the time you’re ready to use it.

Preheat oven to 400°F (204°C). Line a 12-cup muffin tin with paper liners and set it aside.

In a large mixing bowl, combine the flour, baking powder, apple pie spice, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, combine the butter and both sugars and beat on medium-low speed until light and fluffy; about 3 minutes.

Add in the eggs and yolk, one at a time, beating well after each addition. Slowly beat in the vanilla, buttermilk, and sour cream. Reduce mixer speed to low and gradually add in the dry ingredients, mixing just until combined. Be sure not to over mix here.

Turn mixer off, and using a rubber spatula or wooden spoon, gently fold in the apples. 

Divide muffin batter evenly among the muffin cups, then crumble up the topping and evenly distribute it on top of the muffins, piling it high in the center. Gently push the streusel into the batter. This helps it stay put as the muffins bake.

Bake for 22 to 24 minutes, or until the tops are lightly golden brown and a toothpick inserted in the middle comes out clean. Allow the muffins to cool in the pan, placed on top of a wire rack, for 15 minutes, then transfer them to a large serving plate. 

Drizzle a little honey on top of each muffin, if desired, and serve warm.

To store, cool completely, then wrap muffins tightly in plastic wrap, and keep at room temperature, for up to 2 days.