Pre-heat the oven to 350°F. Place a piece of parchment paper on the inside of an 8-inch square baking pan, letting the parchment paper extend an inch over two opposite edges of the pan. Smear a little butter in the corners of the pan to secure the parchment paper to the pan and then butter the two sides of the pan that are not covered with parchment.

Whisk the melted butter and brown sugar together in a large bowl until well combined. Add the egg and vanilla extract.

Stir in the flour, baking powder and salt just until no traces of white remain. Do not overmix. Gently fold in the blueberries. Spread the batter evenly into the prepared pan.

Bake at 350°F for 35 minutes. Transfer the baked blondies to a cooling rack. Run a knife around the edges and allow the blondies to cool completely in the pan.

If you are making the white chocolate glaze to drizzle on top, melt the chocolate chips and butter together in a double boiler (or in the microwave using 10 second bursts at a time and stirring in between) until the chocolate has partially melted. Stir in the hot water (you can use the simmering water from the bottom of the double boiler) and then stir in the powdered sugar, beating vigorously to break up any lumps. Put the white chocolate glaze into a piping bag or into the corner of a zipper sealable plastic bag. Cut off the tip of the plastic bag if using and drizzle the white chocolate glaze over the blondies in a zigzag pattern.

Cut the cooled blondies into 16 squares. Store in an airtight container for a few days or in the refrigerator for up to 1 week.