📖 Instructions

1. **Make the Shortbread Crust:** Preheat your oven to 350°F (175°C). In a large bowl, whisk together the flour, powdered sugar, and salt. Add the cold, cubed butter and use a pastry blender or your fingers to cut the butter into the flour mixture until it resembles coarse crumbs. Don't overwork the dough; you want small pieces of butter visible for a flaky crust.
2. Press the Dough:** Press the mixture evenly into the bottom of an ungreased 9x13 inch baking pan. Use the bottom of a measuring cup to help create a smooth, even surface. Prick the crust all over with a fork. This prevents it from puffing up during baking.
3. Bake the Crust:** Bake the crust for 18-20 minutes, or until it's lightly golden brown. Keep a close eye on it; you don't want it to burn. Remove from the oven and let it cool slightly while you prepare the filling.
4. **Make the Rhubarb Filling:** In a medium saucepan, combine the chopped rhubarb, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the rhubarb softens and releases its juices. This usually takes about 8-10 minutes. The rhubarb should be tender but still hold its shape a bit.
5. Thicken the Filling:** In a small bowl, whisk together the cornstarch and water to create a slurry. Pour the slurry into the rhubarb mixture and stir constantly until the filling thickens. This should only take a minute or two. Remove from heat and set aside to cool slightly.
6. **Make the Elderflower Custard:** In a separate bowl, whisk together the eggs, sugar, elderflower liqueur (if using), vanilla extract, and salt until well combined. Stir in the heavy cream until everything is smooth. The mixture should be pale yellow and slightly frothy.
7. Assemble the Bars:** Pour the rhubarb filling evenly over the pre-baked shortbread crust. Gently pour the elderflower custard over the rhubarb filling. Make sure the custard distributes evenly across the entire surface.
8. Bake the Bars:** Return the pan to the oven and bake for 25-30 minutes, or until the custard is set and the top is lightly golden brown. The center should be just slightly jiggly but not liquid. If the crust starts to brown too quickly, you can tent the pan with foil.
9. Cool Completely:** Remove the bars from the oven and let them cool completely in the pan before cutting into squares. This is important because the custard needs time to set fully. I know it's hard to resist, but trust me, it's worth the wait. If you try to cut them while they're still warm, they'll be a gooey mess.
10. Chill (Optional):** For even cleaner cuts and a firmer texture, chill the bars in the refrigerator for at least an hour before serving. This also allows the flavors to meld together even more beautifully.
11. Serve and Enjoy:** Cut the cooled bars into squares and dust with powdered sugar, if desired. Serve and enjoy the taste of spring! These are fantastic on their own, but a scoop of vanilla ice cream or a dollop of whipped cream would certainly not be unwelcome.