Preheat the oven to 350°F/180°C/Gas 4. Line a 9x9-inch square baking pan with a piece of parchment paper, leaving some overhang to lift the cooked blondies out of the pan.

Combine butter and 1½ cups of brown sugar in a large mixing bowl. Whisk until smooth and lightly fluffy.

Add the eggs and vanilla extract to the bowl and continue whisking until smooth and combined.

Sift the flour, baking powder, cinnamon, and salt into the bowl. Gently fold until combined, taking care not to overmix.

Toss the apples with the remaining brown sugar in a small bowl. Add the apples and white chocolate chips (if using) to the batter and gently fold in.

Pour the batter into the prepared baking pan and place into the oven to bake for 30-40 minutes, until the top is lightly browned and a toothpick inserted into the center of the pan comes out mostly clean.

Remove the blondies from the oven and leave them in the pan to cool. They will continue to set as they cool down. When the blondies have cooled to room temperature, lift them from the pan, slice, serve, and enjoy!