Preheat your oven to 350 degrees F. Line a 9x13 pan with tin foil and spray with cooking spray.
In a medium microwavable bowl, melt the butter. Add the flour and powdered sugar. Mix until combined and dough forms.
Drop large spoonfuls of dough into the pan and press into the bottom of the pan using your hands. Do not press it up the sides but make sure it touches all sides and corners.
Bake for 20 minutes. Edges should be just lightly brown. Remove from oven and set aside.
While base is baking, beat eggs in a medium bowl. Add sugar, baking powder, lemon juice and flour. (You can add more or less lemon juice depending on how tart or sweet you want the bars to be.)
Pour over the base and bake again for 20-25 minutes at 350 degrees F. The edges should be just lightly brown. Remove bars from the oven and let cool completely.
When cool, sprinkle with powdered sugar over the top, cut and serve.