In a small pot, heat the rhubarb, sugar, and 1/2 cup of water on medium heat, stirring frequently. Cook for 10-12 minutes, until the rhubarb has completely broken down and looks like apple sauce. If the mixture starts to get dry and stick to the bottom of the pot, add a tablespoon of water as necessary. Remove from heat and pulse in a blender until pureed (it should be mostly pureed, but this will get out any strings of rhubarb left). Set aside.
Make the donuts
Preheat oven to 350°F. Grease a standard sized donut pan and set aside.
In a medium bowl, combine flour, sugar, baking soda, baking powder, and salt. In a separate bowl, combine egg, buttermilk, vegetable oil, 1/4 cup of the rhubarb puree, vanilla extract, and orange zest. Carefully pour the wet ingredients into the dry, mixing until just combined. Mixture will be slightly lumpy and thick.
Spoon batter into a large ziplock bag and cut a ½ inch hole in one corner, creating a makeshift piping bag. Pipe the dough into the donut molds, filling ¾ of the way to the top of the molds. Bake for 12 minutes. Allow to cool in fridge for 10 minutes before glazing.
To make the glaze, combine powdered sugar, 1/4 cup rhubarb puree, and whole milk in a small bowl, whisking until smooth. The consistency should be thick but still thin enough to form ribbons when you scoop it from the bowl and back down. Stir in 1-2 drops of red food coloring, depending on how pink you want your donuts.
Dip each donut into the glaze and garnish a corner with orange zest. Serve!