Preheat oven to 350° F. Grease and flour a 9 x 13-inch glass baking dish or line the bottom and sides with parchment paper, leaving an overhang on the sides.
Prepare Crust:
In a large bowl, stir together melted butter, flour and confectioners sugar until completely combined.
Press the mixture into the prepared baking dish. I like to use the flat bottom of a measuring cup to gently press the crust into an even layer. Bake until golden brown, about 15-20 minutes.
Prepare Filling:
While the crust is in the oven, make the filling. In a large bowl, whisk together the sugar, flour and baking powder. Add the eggs, lemon juice and lemon zest; whisk until completely combined.
Pour the filling over the hot crust. Return to the oven and bake for 25 minutes.
Cool completely on a wire rack. Dust with additional powdered sugar.
Slice into squares and serve.
Notes
Use a glass baking dish if possible, or a ceramic dish as an alternative. Metal pans can sometimes give the bars a metallic aftertaste.
Use freshly-squeezed lemon juice (not bottled juice) for the best flavor.
When zesting the lemon, be sure that you’re only scraping off the outer layer of the peel (this is where the essential oils and delicious flavor can be found). Be careful not to grate any of the bitter white pith from the fruit.
Allow the bars to cool completely before dusting with sugar and slicing. I like to cool the bars at room temperature for about 1 hour, and then refrigerate the pan for at least 2 more hours before cutting.
If you notice little white bubbles on the top of your bars after baking, that's totally normal. These little air bubbles rise to the top as the eggs cook. Just dust the cool bars with powdered sugar and the bubbles will be undetectable.