Yield: 8-inch square pan
Preheat oven to 350 degrees F. Line an 8-inch square baking pan with parchment paper, allowing for an overhang on all four sides (this will help in unmolding after baking). Lightly grease and set aside.
In the bowl of stand mixer fitted with a paddle attachment (or using an electric hand mixer), beat together softened butter and brown sugar. Mix until smooth and fluffy, about 2-3 minutes. Scrape down bowl as needed to ensure thorough mixing. Add egg and vanilla. Continue to mix well combined.
In another bowl, whisk together flour, baking powder, baking soda, and salt. Add flour mixture to butter mixture and beat to combine. Fold in white chocolate chips and chopped apples.
Transfer batter to prepared pan and press down into an even layer. Bake for 25-30 minutes until toothpick inserted in center of pan comes out clean. Allow to cool in pan for 15 minutes. Lift out the blondies using the parchment overhang. Transfer to cooling rack to cool completely. Alternatively, store in the fridge until chilled. Once cooled, slice into desired serving pieces.
Leftover blondies can be stored at room temperature in an airtight container overnight. For longer storage, keep in an airtight container in the fridge. Warm to room temperature before eating. Will keep in the fridge for up to 4 days.