01 - Preheat oven to 400°F. Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - In a large mixing bowl, whisk together flour, granulated sugar, baking powder, salt, and cinnamon until evenly blended.
03 - Add cold cubed butter. Use a pastry cutter or fork to work butter into flour mixture until it resembles coarse crumbs.
04 - Beat eggs, milk, and vanilla extract together in a separate bowl. Pour wet mixture over dry ingredients and gently stir just until combined; avoid overmixing.
05 - Gently fold diced tart apple into batter to distribute evenly.
06 - Divide thick batter equally among prepared muffin cups.
07 - Bake 20 to 22 minutes, or until golden brown and a toothpick inserted in the center comes out clean.
08 - Allow muffins to cool in the pan for five minutes, then transfer to a wire rack.
09 - Place brown sugar, butter, and milk in a small saucepan over medium heat. Stir constantly, bringing to a simmer until sugar dissolves, about two minutes.
10 - Remove saucepan from heat. Mix in vanilla extract and salt, then whisk in sifted powdered sugar until smooth.
11 - Drizzle warm caramel glaze over cooled muffins. Let glaze set for ten minutes before serving.