Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Whisk melted butter with brown and granulated sugars until smooth and glossy.
Whisk in the egg and vanilla until fully combined.
Combine flour, baking powder, salt, cinnamon, and nutmeg in a bowl.
Fold dry ingredients into the wet mixture just until no flour streaks remain.
Stir in diced apples and pecans to evenly distribute.
Spread batter into the pan and smooth the top. Tap the pan to level.
Bake 25–30 minutes, until set and a toothpick shows a few moist crumbs.
Cool in the pan 20 minutes, then lift out to a rack to cool completely.
Whisk powdered sugar, maple syrup, melted butter, and vanilla. Thin with milk to a pourable glaze; season with a pinch of salt.
Drizzle glaze over cooled blondies. Let set 15 minutes, then cut into 16 squares.