For the shortbread tart shell: Toss the flour and salt together on your work surface. Rub the butter into the flour with your fingertips (or use a pastry cutter). When there are only little shreds of butter remaining, make an indentation in the center of the flour and put the powdered sugar, vanilla or vanilla sugar, egg yolks and lemon zest into the center. Using a fork, mix the egg yolks with the powdered sugar, making a thick paste in the center of the flour. Bring all the ingredients together with your hands and knead until smooth dough is formed. Wrap dough in plastic wrap and chill in the refrigerator while you preheat the oven and chop the rhubarb.

Preheat oven to 400°F (200°C). Grease an 11 inch (28 cm) round tart pan.*

Take the dough out of the refrigerator and press it evenly into the bottom and sides of the tart pan with your fingers. OR: roll out the dough, ease it into the pan and cut off the overhanging dough. (Bake the dough scraps on a cookie sheet and eat them.)

Thinly spread jam over the bottom of the tart case. Fill the tart with the chopped rhubarb. Bake for 25 minutes, until the shortbread is browned and the rhubarb softened. You may reduce the baking temperature to 350°F (170-180°C) after 10-15 minutes if the shortbread is browning too quickly. Remove the tart from the oven and allow it to cool slightly while you prepare the meringue.

For the meringue: Beat the egg whites and salt until foamy. Gradually sprinkle in the sugar and beat on high until very stiff peaks form. Meringue should be thick and glossy. Spread meringue over the tart and use the back of a spoon to make decorative peaks in the meringue. Return the tart to the oven (400°F, 200°C) and bake until the meringue is nicely browned or grill/brown the meringue from above if your oven has this function. Watch meringue constantly to prevent it from burning! Remove tart from the oven and allow it to cool completely on a wire rack before slicing.