In a bowl, add the flour and sugar. Cut the butter into the flour/sugar mixture until the mixture resembles coarse crumbs.
Press into a greased 8x8-in. baking pan. Bake at 350° for 10 minutes.
For Filling:
Combine sugar and flour in a bowl. Whisk in cream, vanilla, zests, and eggs. Stir in the rhubarb. Pour over crust. Bake at 350° until custard is set, 30-35 minutes.
Cover and chill. When ready to serve dust with powdered sugar, and cut into bars. Store in the refrigerator.