Preheat your oven to 350°F. While it heats, chop the rhubarb into 1/2-inch pieces and set aside—uniform sizing ensures even cooking. Line an 8x8 inch baking dish with parchment paper, leaving some overhang for easy removal later. Sift the powdered sugar into a small bowl to remove any lumps before dusting.

In one bowl, whisk together flour, baking powder, and salt. In a separate bowl, beat the vegan butter and cane sugar together until light and creamy, about 2-3 minutes. Add the flaxseed, vanilla paste, and almond milk to the butter mixture and mix until fully combined. I like to use ground flaxseed rather than whole seeds—it distributes more evenly and creates a better texture in the crust.

Fold the dry ingredient mixture from Step 2 into the wet mixture until just combined—don't overmix, as this can make the crust tough. Press the dough evenly into the prepared 8x8 baking dish and bake for 20 minutes until lightly golden and set. The crust should be firm but not browned at this stage since it will bake again with the filling.

While the crust bakes, combine the chopped rhubarb, sugar, and water in a pot and bring to a boil over medium-high heat. Reduce heat and simmer for 5-8 minutes until the rhubarb is very soft and breaking apart. Pour the cooked rhubarb mixture (including all juices) into a blender, add the coconut cream, and blend until completely smooth. I prefer Thai Kitchen coconut cream because its thick consistency helps the filling set properly without becoming too runny.

Return the rhubarb puree to the pot and sprinkle the arrowroot powder over the top, stirring constantly to prevent lumps. Cook over medium heat for 3-5 minutes, stirring frequently, until the mixture noticeably thickens and coats the back of a spoon. The arrowroot will activate and gel the filling, giving the bars a pleasant, sliceable texture.

Remove the pre-baked crust from the oven and spread the thickened rhubarb filling evenly over the top. Return to the oven and bake for an additional 15 minutes until the filling is set but still slightly jiggly in the very center—it will continue to firm up as it cools. This second bake helps the flavors meld and prevents a soggy crust.

Allow the bars to cool completely in the pan for at least 1-2 hours (or overnight for easier slicing). Once fully cooled, lift out using the parchment overhang and cut into squares. Just before serving, dust generously with the sifted powdered sugar from Step 1.