Preheat oven to 350℉. Melt the butter and prepare to measure the rest of your ingredients as it cools.
1/2 cup unsalted butter
Grease the pan. For easy removal of the cake, place parchment paper on the bottom and 2 sides of the pan. Grease again and then set aside
In a small bowl, make the streusel. Combine flour, white sugar, and cold cut up butter. Using your hands, mix it together until combined into a crumbly, streusel mixture. Set aside in the refrigerator while you make the blondie batter.
7 tbsp butter cold, 1 cup all-purpose flour, 1/3 cup white sugar
In another small bowl, add in flour, salt, baking soda, and baking powder. Using a spatula, mix around until mixed together.
1 cup all purpose flour, 1/2 tsp baking powder, 1/4 tsp baking soda, 1/2 teaspoon salt
In a large bowl, whisk together butter and brown sugar until well combined. Then add in lemon zest, egg, and vanilla and keep on whisking.
3/4 cups brown sugar, 1 large egg, 1 tbsp vanilla extract, 1 tbsp lemon zest
Add the dry ingredients into the wet and using a spatula, gently fold until a soft blondie batter forms. Don't overmix!
In the bowl with the blueberries, add 1-2 tsps of flour and gently toss around until evenly coated. The light coating of flour will absorb any extra liquid it may have before adding it into the mixing bowl.
3/4 cup blueberries
Spread evenly into the pan. Top off with more blueberries and streusel.
Bake for 25-29 minutes or until the edges are a golden brown colour. Allow to cool, slice, and enjoy!