Preheat your oven to 325 degrees F. Stick a medium stainless-steel or glass bowl in the freezer (we're going to use this to beat the egg whites later on in the process). Liberally grease your doughnut pan with cooking spray. Note: I NEVER use cooking spray but I've found it necessary when making this recipe. Set aside.
In a small bowl, add the sugar and vanilla bean caviar. Using your hands, vigorously rub the vanilla beans into the sugar so it's evenly dispersed. In a medium bowl, add the vanilla sugar, all-purpose flour, baking powder, baking soda and salt.
In a large bowl, whisk together the egg yolks, vegetable oil, water and vanilla extract. Next, in two batches, add the dry ingredients and mix until you no longer see any flecks of flour, and then mix in the poppy seeds.
Remove the bowl from the freezer, which by now should be very cold, and add the egg whites and pinch of salt. Using a hand-mixer with the whisk attachments, beat the egg whites until stiff peaks form, about 3 minutes. Gently fold the stiff egg whites into the batter, being sure to not deflate the egg whites by going too quickly.
Using a teaspoon, spoon the batter into each of the cavities in the doughnut pan, filling each one almost all the way. Transfer the pan the oven to bake for 8 to 10 minutes, until the edges are lightly golden brown and the doughnuts have risen. Run a sharp paring knife along the edge of each doughnut and invert onto a cooling rack. Repeat the process until you've baked your way through all of the batter.
To make the glaze, add the rhubarb and water to a small saucepan. Cover the pot and place over medium heat, simmering the mixture for about 5 minutes. After the 5-minute mark, uncover the pan and using the back of a wooden spoon or spatula, press the rhubarb down. Strain the rhubarb, pressing on it to expel all of the rhubarb water; discard the rhubarb pulp and reserve the rhubarb water. Measure out 1 tablespoon. To a medium bowl, sift in the powdered sugar and pour in the 1 tablespoon of rhubarb syrup; whisk until very smooth. If needed, add a splash more of rhubarb syrup, but be careful, you want the glaze to be thick.
To assemble the doughnuts, dunk each doughnut into the glaze and garnish with a sprinkling of poppy seeds. Repeat until all of the doughnuts are covered in pretty glaze and a poppy seeds.