Preheat oven to 350°F (175°C) and line a 9x9 inch baking pan with parchment paper.

In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.

Cut the cold butter cubes into the flour mixture using a pastry blender or fork until the mixture resembles coarse crumbs.

Stir in the beaten egg until the mixture is evenly moistened and remains crumbly.

Press approximately half of the crumb mixture firmly into the bottom of the prepared pan to form an even base layer.

In a separate medium bowl, whisk together the sweetened condensed milk, lemon juice, lemon zest, and egg yolk until the mixture is smooth and slightly thickened.

Pour the lemon filling over the base layer, spreading it evenly to the edges.

Distribute the remaining crumb mixture evenly over the top of the lemon layer.

Bake for 40 to 45 minutes or until the edges are lightly golden and the center is set.

Cool the bars completely in the pan on a wire rack, then refrigerate for at least 2 hours before slicing into 16 squares.