Preheat oven to 375°F and grease a 12-cup muffin tin or line with paper liners.
Mix together flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
In a large bowl or the bowl of a stand mixer, cream together butter and sugar. Beat in maple syrup, eggs, and vanilla until smooth. Stir in milk. In another bowl, beat together butter, sugar, eggs, and syrup until smooth. Stir in milk.
Gradually add flour mixture to wet ingredients, mixing just until combined and no dry bits remain. Fold in diced apple.
Divide batter equally among muffin cups. Bake for 20 minutes, until a toothpick inserted in the center comes out clean.