Preheat the oven to 350℉. Prepare your pan by greasing it and lining it with parchment paper, leaving enough overhang to grab later.
Stir the flour and baking powder together and set aside.
Mix the melted butter, sugar, and lemon zest, beating until the sugar is mostly dissolved. Beat in the eggs one at a time, mostly mixing each before adding the next. Finally, add in the lemon juice and extract (if you're using it) and fully mix it all.
Add in the flour mixture and fold it all to combine it, but don't over-work it.
Spread the batter evenly in the pan. It will be thick so you may need to nudge it with your spatula.
Bake for 27 or so minutes. The edges will turn golden and a toothpick should be clean after inserting it in the middle.
Mix together your icing by just mixing up the ingredients. Once the lemonies are mostly cool, spread the icing over the top and let the last residual heat coat it all. Then let it finish cooling, remove from the pan, cut, and serve.