For the cookie bars:

• Preheat oven to 350°F. Line 8x8-inch square baking pan with parchment paper.
• In bowl of stand mixer fitted with paddle attachment (or in large bowl if you plan to use a hand-held mixer) add sugar and lemon zest. Use your hands to rub the lemon zest with the sugar, allowing the lemon oils to release and absorb into the sugar. This step adds so much lemon flavor!

  1 cup (200 grams) granulated sugar, 1 tablespoon lemon zest
• Add the butter and beat with the lemon sugar until smooth and creamy, about 2-3 minutes. Scrape down bowl and add eggs and yolk one at a time, mixing after each.

  10 tablespoons (140 grams) salted butter, 2 large eggs, 1 large egg yolk
• Combine flour, salt, and baking powder in a separate bowl. Add half of the mixture to your mixer bowl and mix on low until combined. Add the lemon juice, followed by the remaining dry ingredients, mixing after each. Don't overmix, stop when cookie dough is smooth and no dry ingredients remain.

  2 cups (260 grams) all-purpose flour, ½ teaspoon baking powder, ½ teaspoon salt, ¼ cup freshly squeezed lemon juice
• Spread batter evenly into prepared pan and bake until edges just turn golden brown, about 25-30 minutes. Let bars cool completely.
• Whisk together powdered sugar, lemon zest and enough lemon juice to make a thick but spreadable icing. Pour over cooled bars and carefully spread to the edges.
• Let sit for at least 30 minutes for the icing to set up before slicing, though icing will still be somewhat soft. (If you plan to stack or transport bars, I recommend letting them set up for several hours so the frosting dries out completely before slicing.)
• Store lemon sugar cookie bars in an airtight container for up to 3 days. After that they start to dry out.