For the shortbread: Preheat oven to 350° and butter a small gratin dish (any variation on the 8- by 8-inch size will do). Place all of the ingredients for the shortbread in a bowl and knead the butter into the dry ingredients with your fingers. When it has the consistency of moist breadcrumbs and all the butter is incorporated, pour into the gratin dish and press down into the bottom. Put in the oven and bake for about 15 minutes or until it just begins to brown. Remove from the oven.
For the filling: Put the rhubarb in a saucepan with the maple syrup and place over medium heat. Let the mixture simmer and break down until the rhubarb has “melted” into a purée and most of the liquid is boiled off. Set aside.
In a medium-sized pot put the yolks, sugar, zest, juice and butter. Stir with a whisk over medium-low heat until the sugar has dissolved and it starts to thicken (about 5 minutes). Add the rhubarb purée and cook an additional 3 minutes over low heat, stirring constantly, being careful not to let it scorch. Pour the mixture over the shortbread and return to the oven for 8 minutes. Remove when the rhubarb purée has just begun to set. Let cool, cut into squares and dust with some confectioner’s sugar if you wish.