Preheat the oven to 350 degrees F and line a 9x9-inch square baking pan with a piece of parchment paper.
In a large mixing bowl, combine butter and 1 ½ cups of brown sugar. Whisk until smooth and lightly fluffy.
Add in the eggs and vanilla extract; continue whisking until smooth and combined.
Add in the flour, baking powder, cinnamon, and salt. Gently fold until the batter and smooth, taking care not to overmix.
Toss apples with remaining brown sugar in a small bowl. Gently fold the apples and the white chocolate chips, if using, into the batter.
Transfer the blondie batter into the prepared baking pan and place in the oven to bake until the top is lightly browned and a toothpick inserted into the center of the pan comes out mostly clean, around 30-40 minutes. The blondies will set as they cool down.
Allow the blondies to cool to room temperature before slicing and serving. Enjoy!