Preheat oven to 350 degrees and line a 9x9 baking pan with parchment paper. For thicker bars, use an 8x8 pan.
In a large bowl, cream butter and sugar together until fluffy. Whisk in eggs and egg yolk until completely combined. Stir in lemon juice and lemon zest. Add flour and salt and mix until just combined - do not over mix. Pour batter into prepared baking pan and bake for 20-25 minutes or until the edges just begin to brown and the top of the bars don't look "wet" anymore.
While bars are baking, prepare glaze by whisking powdered sugar, lemon juice and optional lemon zest together until smooth. Add lemon juice as needed to achieve the desired consistency. (You want it to be semi-thick since you are spreading it on warm bars and it will thin out a bit.) Once bars are done, allow them to cool slightly, about 5 minutes, then spread the glaze on top with a rubber spatula. Once glaze is set and bars are cooled completely, cut them into bars and serve.
Lemonies will stay fresh for up to 4 days stored in an airtight container.