Preheat your oven to 350°F (175°C). Lightly grease a 9x9-inch baking dish with butter or nonstick spray to prevent sticking.

In a small bowl, toss the fresh blueberries with ¼ cup of flour until coated. This helps prevent them from sinking to the bottom of the cake while baking. Set aside.

In a large mixing bowl or stand mixer, cream together the softened butter, 1 cup of sugar, and lemon zest for 3–5 minutes, or until the mixture is fluffy and lightened in color. This step incorporates air, giving the cake a tender texture.

Beat in the egg until fully incorporated, then stir in the vanilla extract.

In a separate bowl, whisk together the remaining flour, baking powder, salt, and cinnamon (if using).

Alternating between the flour mixture and buttermilk, add both to the batter, beginning and ending with the dry ingredients. Mix until just combined to avoid overworking the batter.

Gently fold the floured blueberries into the batter. Pour the batter into the prepared baking dish, spreading it evenly. Sprinkle the top with 1 tablespoon of sugar for a subtle crunch.

Bake for 40–50 minutes, or until a toothpick inserted into the center comes out clean.

Transfer the cake to a wire rack and let it cool for at least 15 minutes before slicing. Serve warm or at room temperature.