1. PREPARE THE PAN AND OVEN: Preheat the oven to 350°F. Grease a 9×13-inch baking dish and line it with parchment paper, leaving a 2-inch overhang on each of the two long sides to create a sling for easy removal. Whisk together the flour, baking powder, baking soda, and salt in a medium bowl until well combined. Set aside.

2. CREAM BUTTER AND SUGAR: Cream the softened butter and sugar together in a large bowl using a stand mixer or hand mixer on medium speed for 3–4 minutes until the mixture turns pale, light, and fluffy. Scrape down the sides of the bowl halfway through. This creaming step creates the air structure that produces a tender crumb — do not rush it.

3. ADD WET INGREDIENTS: Add the eggs and vanilla extract and mix on low speed until incorporated. Add the buttermilk and mix on low until just combined — the batter may look slightly curdled at this stage, which is normal. Add the lemon juice and zest and mix briefly until blended.

4. ADD DRY INGREDIENTS AND BLUEBERRIES: Add the flour mixture and mix on low speed just until no visible dry flour remains — do not overmix or the cake will be tough. Toss the fresh blueberries with 1 tablespoon of flour in a small bowl until lightly coated. Use a rubber spatula to gently fold the flour-coated blueberries into the batter with 3–4 light strokes.

5. BAKE: Pour the batter into the prepared 9×13 pan and spread it evenly with the spatula. Bake for 40–45 minutes until a toothpick inserted into the center comes out with just a few moist crumbs — not wet batter. The top should look golden and spring back lightly when pressed. Cool the cake completely in the pan on a wire rack before glazing.

6. MAKE AND POUR THE LEMON GLAZE: Whisk the powdered sugar and fresh lemon juice together until completely smooth and pourable. Once the cake is fully cooled, pour the glaze over the top and tilt the pan gently to spread it toward the edges. Let the glaze set and harden for 30 minutes before slicing. Garnish with a few fresh blueberries or a sprinkle of lemon zest if desired.