In a saucepan, brown the butter until golden and nutty. Stir in apple butter and 1 cup brown sugar.
Whisk in eggs and vanilla. Add flour, baking powder, 1 teaspoon cinnamon, nutmeg, and salt. Stir just until combined.
Toss diced apples with remaining brown sugar and 1 teaspoon cinnamon.
Spread batter into a greased baking dish. Sprinkle apples evenly on top.
Bake at 350°F for 30–35 minutes, until golden and set. Let cool completely.
For the glaze: brown the butter again, then whisk in maple syrup, powdered sugar, cinnamon, and salt until smooth.
Drizzle glaze over cooled blondies. Let set, slice, and serve.