In medium size bowl mix 3 Tbsp sugar and flour. Cut in butter until mixture is crumbly. Press crumbs firmly and evenly into bottom of an 8x8 inch square pan.
Bake for 15 to 17 minutes until just starting to brown.
To prepare lime filling, beat together eggs, 2/3 cup sugar, 2 Tbsp flour, 1/4 tsp baking powder, lime zest and lime juice.
Pour filling over baked pastry layer.
Bake for 20 minutes or until lightly golden around edges and center is set.
Cool completely on a wire rack.
Cut into 16-20 squares/bars.
Sift powdered sugar over top of bars and garnish with extra lime zest.
Notes
Try switching the lime for lemon or even blood oranges! No need for green food coloring - the bits of green zest and the incredible flavor speak for itself.