Lightly grease muffin pan with cooking spray or line with cupcake liners.
Make cinnamon crumble by mixing together sugar, cinnamon, flour, and cubed butter in a small bowl. Use a fork or pastry blender to create crumble (butter pieces should be small). Set aside.
Cream butter, sugar, vanilla, and salt in a large bowl until fluffy. Add eggs one at a time and beat until mixed.
In another bowl, mix flour, baking powder, and cinnamon. Add this to your creamed mixture, along with the milk, and mix until just incorporated. Fold in sliced apples.
Scoop into muffin cups until ¾ filled. Sprinkle with crumble and bake for 28-30 minutes, or until muffin tops are golden brown and a toothpick inserted in the center comes out clean. Let cool before serving.
Notes
STORE. Place cooled muffins in an airtight container or Ziploc freezer bag. Store at room temperature for 1-2 days, in the fridge for 3-4 days, or in the freezer for 2-3 months. Thaw if needed before serving.