Preheat the oven to 350°F (175°C). Grease and line an 9x9-inch (or similar square) baking pan with parchment paper, leaving an overhang for easy removal.

Combine dry ingredients

In a medium bowl whisk together the all-purpose flour, baking powder, and salt until evenly distributed. Set aside.

Whisk wet ingredients

In a large bowl, whisk the granulated sugar and eggs until combined and slightly thickened (about 1 minute). Whisk in the Greek yogurt, melted cooled butter, vanilla extract, and lemon zest until smooth.

Fold batter

Add the dry ingredients to the wet mixture in two additions, folding gently with a spatula until just combined. Do not overmix — a few small streaks are fine. The batter will be thick but pourable.

Blueberry Layer & Assembly

Macerate blueberries

In a medium bowl toss the fresh blueberries with 2 tablespoons granulated sugar, 1 tablespoon lemon juice and the optional 1 teaspoon cornstarch until evenly coated. Let sit 8–10 minutes; juice will form and the cornstarch will absorb it slightly, giving a glossy, slightly thickened coating.

Fold some blueberries into batter

Reserve about 3/4 cup of the macerated blueberries for the top. Gently fold the remaining blueberries into the batter, distributing them evenly but taking care not to crush them.

Pour and level

Pour the batter into the prepared pan and smooth the top with an offset spatula or back of a spoon so it is evenly distributed.

Top with blueberries

Scatter the reserved macerated blueberries evenly over the surface of the batter. If any liquid accumulated from macerating, spoon it sparingly over for extra fruit pockets, but avoid adding too much or it may sink excessively.

Bake

Bake in the preheated oven for 30–38 minutes, or until the top is golden and a toothpick inserted into the center (avoiding large blueberry pockets) comes out with only a few moist crumbs. Rotate the pan halfway through baking for even color.

Cool in pan

Remove from oven and set on a wire rack. Let the cake cool in the pan for 15 minutes, then use the parchment overhang to lift it out and transfer to the rack to cool completely (about 45–60 minutes) before glazing and slicing. Cooling prevents the glaze from melting and the cake from breaking.

Topping / Glaze (optional)

Make glaze

Whisk the powdered sugar with the lemon juice and 1 teaspoon milk or water in a small bowl until smooth and pourable. Add an extra 1/4 teaspoon liquid if needed to reach desired consistency.

Glaze and garnish

Once the cake is completely cool, drizzle the glaze over the top using a spoon or small piping bag. Sprinkle the grated lemon zest over the glaze for brightness. Allow glaze to set 10–15 minutes before cutting into squares.

Serve

Slice the cooled, glazed cake into pieces (about 9-12 squares). Serve at room temperature. Store leftovers covered in the refrigerator for up to 3 days; bring to room temperature before serving for best texture.