For the bars: Preheat the oven to 350 degrees F with a rack in the middle.
Grease and line a 9-inch square baking pan with parchment paper. Cut enough so there are wings hanging over the sides of the pan for when it's time to remove the bars!
In a food processor, combine the flour, almond flour, granulated sugar and salt and pulse to combine. Add the butter, egg white and rosewater, if using. Pulse until the mixture clumps together into small to medium clumps of dough.
Pour the dough out into the prepared pan and press evenly all around the bottom with a measuring cup or the back of a spatula until smooth.
Bake until the crust is just set and the edges are light golden, 15 to 18 minutes. Remove to a rack to cool. Reduce the oven temperature to 325 degrees F.
For the filling: Combine the rhubarb in a medium saucepan with water just to cover (about 1 1/2 cups water) and 1/4 cup granulated sugar. Scrape in the vanilla bean seeds and add the pod. Simmer until the rhubarb is falling apart, about 10 minutes. Cool, then strain and discard the solids. (See Cook's Note.)
Add 1/4 cup of the reserved rhubarb juice to a small mixing bowl. Add the cornstarch. Use a small whisk or fork to combine until smooth and there are no cornstarch lumps. Add the rhubarb slurry to the saucepan with an additional 1/2 cup of the reserved rhubarb juice. (See Cook's Note for uses for any extra juice).
Whisk in the eggs, yolks, salt, remaining 1 1/4 cups granulated sugar and 2 tablespoons lemon juice. Cook, whisking constantly, over medium heat until the mixture begins to simmer and thicken and bubbles pop to the surface, 7 to 10 minutes. Whisk in the zest, then the butter and coconut oil a little at a time. Whisk in just enough food coloring to make a salmon pink color, 2 to 3 drops.
Pour the filling mixture over the cookie crust and bake until the edges are set, but the center is just a tiny bit jiggly, 25 to 30 minutes. Remove to a rack to cool completely.
Once cooled, remove the bars from the pan. With a sharp knife, cut into 16 squares, wiping the blade after each cut. (You can also trim the edges for a cleaner look.) Dust with powdered sugar before serving.
For longer storage, refrigerate between layers of parchment in a sealed container.
You can save and dry out the vanilla bean pod. Add to a canister of sugar or tin of whole coffee beans to give it a mild vanilla flavor.
Use any leftover rhubarb syrup to sweeten ice tea or make cocktails.