Prepare your pan and oven. Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving some overhang for easy removal later.
Make the shortbread crust. In a food processor, combine flour, powdered sugar, and salt. Pulse to mix. Add the cold vegan butter cubes and pulse until the mixture resembles coarse crumbs. Add the tablespoon of cold water and pulse just until the dough comes together.
Press and bake the crust. Transfer the dough to your prepared pan and press it evenly into the bottom. Use the bottom of a measuring cup to flatten it smoothly. Prick the crust all over with a fork to prevent bubbling. Bake for 20-22 minutes until just barely golden at the edges.
Prepare the lemon filling. While the crust bakes, drain and rinse your soaked cashews. Add them to a high-speed blender along with the lemon juice, lemon zest, coconut cream, maple syrup, sugar, cornstarch, turmeric (if using), vanilla, and salt. Blend on high until completely smooth and creamy, about 2-3 minutes, stopping to scrape down the sides as needed.
Pour and bake the filling. When the crust is done, remove it from the oven but keep the oven on. Pour the lemon filling over the hot crust and smooth the top with a spatula. Return to the oven and bake for 25-30 minutes until the edges are set but the center still has a slight wobble.
Cool completely. Allow the bars to cool at room temperature for 1 hour, then refrigerate for at least 4 hours or preferably overnight to fully set.
Slice and serve. Once fully chilled, lift the bars out using the parchment paper overhang. Dust generously with powdered sugar, slice into 16 squares, and serve. For the cleanest cuts, wipe your knife clean between slices.