Start by preheating your oven to 350°F (175°C). Grease a 9x13-inch sheet pan or line it with parchment paper for easy cleanup.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. This ensures the leavening agents are evenly distributed.

In a large bowl, use an electric mixer to cream the softened butter and sugar until light and fluffy—about 2-3 minutes. This step helps create a tender, airy cake.

Beat in the eggs one at a time, followed by the vanilla, lemon juice, and lemon zest. The mixture may look slightly curdled at this point, but don’t worry—it’ll come together once the dry ingredients are added.

Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Start and end with the dry ingredients, mixing just until combined. Be careful not to overmix, as this can make the cake dense.

Toss the blueberries with 1 tablespoon of flour to prevent them from sinking. Then, gently fold them into the batter with a spatula.

Pour the batter into the prepared sheet pan and spread it evenly. Bake for 28-32 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely in the pan.

In a small bowl, whisk together powdered sugar and lemon juice until smooth. If the glaze is too thick, add a teaspoon of milk at a time until it reaches your desired consistency.

Drizzle the glaze over the cooled cake, spreading it evenly with the back of a spoon or an offset spatula. Slice into squares and serve!