Preheat your oven to 350°F (175°C) and line a 9x13 inch baking dish with parchment paper, leaving some overhang for easy removal.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Beat in the egg, vanilla extract, lemon zest, and lemonade concentrate until well combined.
Gradually add the dry ingredients to the wet mixture, mixing until just combined. Do not overmix.
Spread the dough evenly in the prepared baking dish. Use a spatula to smooth the top.
Bake for 20-25 minutes or until the edges are lightly golden and a toothpick inserted in the center comes out clean.
Allow the cookie crumble to cool in the pan for about 10 minutes before lifting it out using the parchment overhang. Let it cool completely on a wire rack.
Once cooled, dust with powdered sugar before cutting into squares.