Place all 20 Tablespoons of butter in a light-colored skillet over medium heat. (Light-colored helps you determine when the butter begins browning.) Stir or whisk the butter constantly as it melts. Once melted, the butter will begin to foam. Keep stirring. After 5–8 minutes, the butter will begin browning—you’ll notice lightly browned specks begin to form at the bottom of the pan and it will have a nutty aroma. As soon as the butter has browned, immediately remove from heat and pour it into a heat-safe glass bowl or liquid measuring cup. Be sure to scrape out all the browned solids at the bottom of the skillet and include them as well. Transfer 3 Tbsp (43g) of the browned butter to a small bowl and reserve for the icing in step 7. The rest will go into the blondie batter in step 5. Because of moisture lost during the browning process, you will now have less butter than you started with—this is expected. Allow brown butter to slightly cool while you continue.

Meanwhile, return the pan to the stove. (No need to rinse out!) Add the apples, maple syrup/brown sugar, and cinnamon. Stir and cook over medium heat until apples have slightly softened, about 3–5 minutes. Set apples aside.

Preheat oven to 350°F (177°C). Line the bottom and sides of a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides to lift out the cooled blondies (makes cutting easier!). Set aside.

In a large bowl, whisk together the flour, baking powder, salt, cinnamon, and nutmeg.

In a medium bowl, whisk together the brown butter, brown sugar, eggs, and vanilla extract. Pour the wet ingredients into the dry ingredients and stir until combined. Fold in the apples. Batter will be thick.

Spread batter evenly into the prepared pan. Bake for about 35 minutes or until the top is lightly browned and a toothpick comes out *mostly* clean. Remove from the oven and allow the blondies to cool completely in the pan set on a cooling rack.

If the brown butter you set aside for the icing has solidified, warm it in the microwave for a few seconds to melt it back into liquid, then whisk together with the confectioners’ sugar, milk, vanilla extract, and salt. If needed, add more confectioners’ sugar to thicken, or more milk to thin out. Drizzle or spread over cooled blondies.

Lift the blondies out of the pan by gripping the parchment paper overhang; transfer to a cutting board and cut into squares. Store tightly covered at room temperature for up to 2 days or in the refrigerator for up to 1 week.