Prepare CrustPreheat oven to 350˚F. Lightly grease the bottom and all sides of an 8"x8" square pan with vegan butter or shortening. Cut 2 pieces of parchment to 8"x16" which will leave about a 2-inch overhang on all sides of the pan to act as handles to lift out the finished bars. Press in one piece of parchment then lightly grease the top of it before laying the second piece of parchment over it in the opposite direction. Greasing the pan and parchment will keep the parchment from sliding around.

Cream together softened vegan butter and sugar with a stand mixer or hand-held mixer until fluffy, about 2 minutes.

Beat in salt, vanilla and lemon zest.

Than beat in milk.

Add flour and mix until just incorporated, about 30 seconds to 1 minute. Stop and scrape sides of bowl with a rubber spatula as needed.

Press shortbread batter into prepared pan in one even layer.

Bake at 350˚F for about 30 minutes, until top is lightly golden with slightly darker edges.

Prepare FillingPrepare the rhubarb curd while crust bakes. Start by softening rhubarb pieces and strawberries with the lemon juice and sugar over medium-low heat in a saucepan. Cook until juices are released and fruit is softened, about 3-5 minutes.

Add the fruit and juices to a blender along with aquafaba, tapioca flour and powdered agar agar then blend on high until the mixture is puréed and smooth.

Rinse saucepan to get rid of any fruit bits.

Transfer rhubarb pureé back into saucepan over medium-low heat, whisking constantly for 5 minutes to activate the thickening agents from the agar and agar and tapioca flour. Mixture should be thick and smooth.

Remove from heat and whisk in coconut oil and vanilla extract.

Set aside until crust has finished baking.

Assemble Bars:When crust has finished baking, remove the pan from oven and place it on a cooling rack.

Lightly scrape the top of crust with a fork to roughen it up a but. This will help the curd join to the baked crust.

Whisk rhubarb curd one more time then pour and spread the curd over the still warm shortbread crust.

Cool at room temp for about 10 minutes before transferring the bars to the refrigerator to chill and set for at least 4 hours or overnight. *Edited to add: If curd is not setting up firmly then pop the pan in the freezer for firm bars before cutting them.

Remove chilled bars from the baking pan by first loosening the parchment from the sides and then lifting up the parchment paper overhangs.

Transfer to a cutting board then cut chilled bars into squares, rectangles or triangles, wiping the blade with a damp warm towel between each cut.

Dust with powdered sugar right before serving. Serve immediately or keep them refrigerated until ready to serve.

Keep leftovers covered and refrigerated for up to 1 week or freeze in freezer-safe bags or containers for up to 6 months. Defrost in refrigerator before serving.