Preheat the oven to 350°F. Line an 8x8 inch pan with parchment paper, letting it hang over two sides for easy lifting, or grease the pan well.

In a large bowl, whisk together the melted butter, brown sugar, egg, and vanilla until smooth and glossy. Let the butter cool slightly if it is very hot.

Add the flour, cinnamon, nutmeg if using, baking powder, and salt to the bowl. Stir gently with a spatula until just combined and no streaks of flour remain. Avoid overmixing.

Peel and finely dice the apple into pea-sized pieces. Fold in the diced apples.

Spread the batter into the prepared pan, smoothing the top with a spatula so it reaches the corners.

Bake for 25 to 32 minutes, until the edges are set and lightly golden and the center no longer looks wet. A toothpick should come out with moist crumbs, not raw batter.

Cool the blondies completely in the pan on a wire rack. Cooling is important so the glaze sets instead of sliding off.

Whisk the powdered sugar, maple syrup, 1 tablespoon of milk or cream, cinnamon, and a pinch of salt in a small bowl. Add tiny splashes of milk if needed until the glaze is pourable but not runny.

Drizzle or spread the glaze over the cooled blondies. Let it set slightly, then lift the slab from the pan using the parchment and cut into squares. Wipe the knife between slices for clean edges.