Preheat oven to 350 degrees F. Lightly butter the bottom and sides of an 8×8 baking pan; line with parchment paper, leaving a slight overhang (this will help lift the blondies out of the pan later).Toss blueberries with 1 tablespoon of flour. Place remaining flour in another bowl and whisk in buttermilk powder and salt until evenly incorporated; set aside.In a saucepan, melt white chocolate and butter over medium heat. Once melted, add brown sugar and whisk to combine. At first the butter will form a yellow slick on top of the melted sugar. Keep whisking for 2 to 3 minutes until it comes together. It will still be fairly greasy, but should appear creamy and emulsified. Remove from heat and let cool slightly.In a mixing bowl or the bowl of a stand mixer fitted with the whisk attachment, beat granulated sugar with egg and egg yolk until lightened in color, 2 to 3 minutes. With the mixer on low speed, slowly pour in the white chocolate mixture, mixing until completely incorporated. Scrape down the sides of the bowl as needed .Mix in vanilla extract and lemon zest.Add dry ingredients and stir until almost incorporated; add blueberries and 1/4 cup white chocolate pieces and gently fold until evenly distributed and no dry flour remains. Pour into prepared pan. If desired, top with a few more blueberries and white chocolate pieces.Bake for 30 to 35 minutes or until top is golden and crackly and a toothpick inserted near the center comes out clean. Let cool completely. Once fully cool, lightly cover and refrigerate overnight or until ready to serve (at least a few hours, but I recommend chilling overnight for the best texture).For the glaze, combine blueberries, lemon juice and water in a small saucepan. Bring to a simmer and cook for about 5 minutes until the blueberries soften, mashing them with the back of a spoon. Remove from heat and strain juice through a fine mesh sieve. You should have about 2 tablespoons of juice.Sift powdered sugar into a small bowl. Add 1 tablespoon blueberry juice and 1 teaspoon maple syrup. Stir until smooth. If the glaze seems a bit thick, add more blueberry juice, 1/2 teaspoon at a time, until the glaze reaches a drizzleable consistency. Use a fork or whisk to drizzle abstract swirls on top of blondies, or a small squeeze bottle fitted with a small round piping tip for a more uniform design. You might not need all the glaze. Chill briefly to set.Use the parchment paper to lift the bars out of the pan, then cut into squares. Blondies will keep, refrigerated in a single layer in an airtight container for up to three days.