Preheat oven to 350-degrees F.
Line an 8x8 inch baking dish with parchment paper. Allow extra paper to hang over 2 of the sides (if needed, you can spray the bottom with a little non-stick spray so the paper sticks). This will allow for easy removal of the bars. Set aside.
In a large microwave safe bowl, melt the butter in 30-second increments. Allow to cool to room temperature and then add the coconut sugar to the melted butter. Using a hand-held mixer, mix on medium-low speed until a thick caramel like texture has formed (this should take about 2 minutes).
Add the egg, egg yolk, and vanilla. Mix until smooth.
In a separate bowl, whisk together the gluten-free flour, baking powder, salt, cinnamon, and nutmeg. Add ½ of the flour mix at a time to the sugar/butter mix. Using your mixer, mix on low speed until just combined. Do not overmix.
Fold in the green apple, reserving a few pieces to sprinkle over the top before baking.
Spread blondie batter into prepared pan. Batter may be sticky, so wet your hands, or a back of a spoon, to easily spread the batter out. Sprinkle, and lightly push down, the batter with leftover apples.
Bake for 25-28 minutes, or until a toothpick entered in the center comes out the clean the edges are just beginning to lightly brown.
While blondies are cooling, make the glaze. In a small bowl, whisk together the powdered sugar, maple syrup, vanilla, and milk/water. If needed, you can add additional powdered sugar to thicken, or additional maple syrup to thin. You want the glaze to be thick, but pourable.
Drizzle glaze over cooled blondies. Allow to set before removing from pan. Using the parchment paper, gently remove the bars from the pan. Cut into squares and transfer to a serving tray (alternatively, you can serve them directly from the pan).
Enjoy!