• Preheat your oven to 350°F (175°C). Grease a 9 x 13-inch baking pan with nonstick spray or line it with parchment paper.
• Brown the Butter. In a light-colored skillet, melt 1 cup of butter over medium heat. The lighter skillet helps you see how the butter is browning. Continue cooking, stirring often, until the butter foams, turns a medium brown color, and gives off a nutty aroma (about 5-7 minutes). Remove from heat and pour the butter into a heat-proof bowl to cool slightly.
  1 Cup Butter
• In a large bowl, combine the brown sugar and white sugar with the cooled brown butter. Stir until smooth. Add the eggs and vanilla extract, mixing until fully combined.
  1 ½ Cup Brown Sugar, 1/2 Cup White Sugar, 2 Large Eggs, 2 Teaspoon Vanilla Extract
• In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, and nutmeg. Gradually stir the dry ingredients into the wet mixture until combined. Gently fold in the chopped apples.
  1 Teaspoon Cinnamon, ½ Teaspoon Nutmeg, 2 Cups All-Purpose Flour, 1/2 Teaspoon Baking Powder, 1/4 Teaspoon Baking Soda, 2 Cups Chopped Apples
• Spread the blondie batter evenly into the prepared pan, pressing it into the corners. Tap the pan on the counter to release any air bubbles. Bake for 25-30 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean. Let the blondies cool completely on a wire rack.

Prepare the Vanilla Frosting

• In a medium bowl, beat the butter with an electric mixer on medium-high speed until light and fluffy, about 2-3 minutes. Gradually add in the confectioners' sugar, heavy cream, and vanilla extract. Beating until smooth and fluffy. Add more cream or sugar as needed for desired consistency.
  1/2 Cup Butter, 3-4 Cups Confectioners' Sugar, 2-3 Tablespoons Heavy Cream, 1 Teaspoon Vanilla Extract
• Once the blondies have cooled completely, spread the frosting evenly over the top. Slice into squares and serve.