Preheat the oven to 375 degrees F. Line a 9x13 pan (or quarter bakers sheet) with parchment paper. Set it aside.
Mix the butter with an electric mixer on high for 1 minute, until light in color. Add the sugar, egg, vanilla extract, and lemon zest, and mix again.
In a separate bowl combine the dry ingredients and whisk lightly, so the salt and baking powder are well incorporated into the flour. Proceed to pour the flour mixture into the wet ingredients and mix well.
Transfer the dough to the prepared pan and press it down so the dough is spread evenly across the entire pan. Bake in the preheated oven for 20 minutes. Let it cool for 20 minutes before adding the glaze.
Whisk all the lemon glaze ingredients in a bowl and pour it onto the warm cookie bars. Spread it across the cookie bars with a rubber scraper. Let the bars fully cool to set. Cut into 30 squares, serve, and enjoy!
Notes
Store leftover cookie bars in an airtight container at room temperature and they will last up to 3 days. Store in the refrigerator for up to 1 week. Freeze the baked bars without glaze, for up to 3 months. Thaw in the fridge, then add glaze before serving.