Preheat your oven to 350°F and line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a medium bowl, whisk together the softened butter, powdered sugar, and sugar until well combined.
½ cup unsalted butter, ½ cup granulated sugar, ¼ cup powdered sugar
Add the egg, lemon juice, lemon extract, and lemon zest, and mix until smooth.
1 large egg, 1 tablespoon lemon juice, ¼ teaspoon lemon extract, 1 tablespoon lemon zest
Stir in the baking powder, salt, and flour until just combined.
½ teaspoon baking powder, 1 & ¼ cup all-purpose flour, ¼ teaspoon salt
Spread the batter evenly into the prepared pan, it will be sticky but just use an offset spatula to spread it as best as you can. Sprinkle with granulated sugar.
extra granulated sugar to sprinkle on top
Bake for 20 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the bars to cool completely in the pan on a wire rack before slicing.