Preheat oven to 350° F. Grease an 8x8-baking pan with cooking spray and line with parchment paper so it overlaps and hangs over the edges.
In a food processor or by hand, combine the ingredients for the crust and mix until it looks like wet sand. Press into the greased baking pan and bake for 20 minutes or until golden.
While the crust is baking, make the filling. In a stand mixer fitted with a paddle attachment or with a hand mixer, mix together the eggs and sugar until creamy. Add the zest, juice and vanilla, finishing with the flour. Mix until fully incorporated and you can no longer see any flour bits.
Pour filling mixture on top of baked crust and bake for another 20 minutes, or until the filling is set. It should jiggle slightly when you shake it but still be firm. Remove and let cool.
Before serving sift with powdered sugar and slice into squares.
Recipe adapted from Unsophisticatedcook.com