Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan or line it with parchment paper.
In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Mix until well combined.
Add the eggs and vanilla extract to the butter-sugar mixture, and whisk until smooth.
In a separate bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Fold in the diced apples and nuts, if using, until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Allow the blondies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.