Prepare the pan: Heat the oven to 350°F and line an 8-inch square pan with parchment paper.

Mix the dry ingredients: Whisk together the flour, baking powder, and salt.

Cream the butter mixture: Beat the softened butter, sugar, and lemon zest until creamy and light.

Add the wet ingredients: Beat in the egg, lemon juice, and vanilla extract.

Make the batter: Stir in the dry ingredients just until combined, then gently fold in 3/4 cup blueberries.

Bake: Spread the batter in the pan, top with the remaining blueberries, and bake for 28 to 34 minutes.

Cool: Let the blondies cool completely before removing them from the pan.

Glaze: Whisk powdered sugar with lemon juice until thick and pourable, then drizzle over the cooled blondies.

Slice and serve: Let the glaze set for 15 minutes, then cut into 16 squares.