Lightly grease a 9x9 inch microwave-safe baking dish OR line with parchment paper.
CRUST
Whisk flour, sugar and lemon zest together.
Stir in melted butter.
Press crust into prepared dish.
Microwave for three minutes at 80% power. Check to make sure the crust is firm. If not, heat for an additional 30 seconds at 80% power.
Set aside.
FILLING
Beat sugar, lemon zest, eggs, and lemon juice together.
Beat in flour, baking powder and salt and continue beating for 2-3 minutes.
Pour the filling over the crust and microwave for three minutes at 80% power. Check the filling. It should be set with just a little jiggle - much like JELLO. If it hasn't set, microwave for an additional minute at 80% power and check again.
Let the lemon squares cool completely and then refrigerate for several hours or overnight.
Cut into squares, dust with powdered sugar and sprinkle additional lemon zest on if desired.