Preheat oven to 350°F (175°C). Line an 11×13-inch baking pan with parchment paper.
In a large bowl, combine 2 1/2 cups flour, 3/4 cup sugar, and 1/2 tsp salt.
Cube 1/2 cup cold butter into 1/2-inch chunks and add to the flour mixture.
Using a pastry cutter or fingers, work the butter into the flour until the mixture resembles coarse crumbs.
Gradually add 3/4 cup buttermilk while gently mixing with a fork until a dough forms.
Press the dough evenly into the prepared pan. Chill the crust for 15 minutes.
Once chilled, prick the crust all over with a fork to prevent bubbles and bake for 20-22 minutes until golden and firm.
While the crust bakes, prepare the lemon filling: In a mixing bowl, whisk together 2 eggs, 1 1/2 cups sugar, and 1 1/2 cups lemon juice until smooth.
Pour the lemon mixture into the cooled crust and bake for an additional 20-22 minutes, until the filling is set but still slightly jiggly in the center.
Allow the bars to cool completely at room temperature before preparing the glaze.
In a small bowl, whisk 2 cups powdered sugar with 2-3 tablespoons lemon juice until smooth and glossy (adjust to desired consistency).
Spread the glaze over the cooled bars and top with 1/4 tsp flaked salt.
Chill in the refrigerator for 2-4 hours before slicing into 18 bars.