1. Make the Cake Batter:

• Preheat and Prep: Preheat the oven to 375°F (190°C). Spray an 8×8 inch square baking pan with nonstick spray and line with parchment paper, leaving an overhang for easy removal. Set aside.   
• Combine Dry Ingredients: In a medium bowl, whisk together the 2 cups all-purpose flour, baking powder, salt, and lemon zest (if using). Set aside.
• Cream Butter and Sugar: Add the softened butter to the bowl of a stand mixer fitted with the paddle attachment (or use a large bowl and hand mixer). Cream the butter on medium-high speed for 1-2 minutes. Add the granulated sugar and continue to cream together for another 2 minutes, or until light and fluffy. Scrape down the sides of the bowl as needed.
• Add Wet Ingredients: Add the room-temperature egg, vanilla extract, and room-temperature Greek yogurt. Beat until combined. Reduce the speed to low and gradually stream in the room-temperature milk until the mixture comes together. (Note: The mixture might look slightly curdled at this point, which is okay).
• Combine Wet and Dry: With the mixer on low speed, gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.   
• Prepare and Fold in Blueberries: In a separate small bowl, toss the blueberries (fresh or frozen) with the 1 tablespoon of all-purpose flour until lightly coated (this helps prevent sinking). Gently fold the floured blueberries into the batter using a rubber spatula. The batter will be thick.

2. Make the Crumb Topping:

• In a small bowl, whisk together the melted (and slightly cooled) butter, granulated sugar, brown sugar, cinnamon, and pinch of salt until smooth.
• Add the 1/2 cup of all-purpose flour and stir with a fork until the mixture is moistened and forms coarse crumbs resembling wet sand.

3. Assemble and Bake:

• Transfer Batter: Transfer the batter to the prepared pan, smoothing out the top evenly with a rubber spatula.
• Add Topping: Using your fingertips, crumble the topping mixture evenly over the surface of the batter.
• Bake: Bake for 40-45 minutes, or until the crumble topping is golden brown, the center of the cake is set, and a wooden toothpick inserted into the center comes out clean.
• Cool: Let the cake cool completely in the pan on a wire rack before removing and slicing. Cooling completely allows the cake to set and prevents crumbling.

4. Serve: Once cool, use the parchment paper overhang to lift the cake from the pan. Cut into 9 squares. Finish with a generous dusting of powdered sugar, if desired.