Preheat the oven to 350 degrees F. Lightly spray a donut pan with nonstick cooking spray and set aside.
Sift together the flour, sugar, baking powder, beet powder, salt and cinnamon in a large bowl.
In a small bowl, whisk together the milk, melted butter, vanilla extract and egg. Add to the flour mixture and stir until just combined.
Using a pastry bag or a plastic sandwich bag with a corner snipped off, pipe the batter into the donut pan cavities. Fill each cavity about ¾ full and be sure to wipe any off that gets dripped onto the center stems. Bake for 12-14 minutes. The tops should be set. When they’re cool enough to handle, remove them from the donut pan and let them cool completely on a wire cooling rack.
for the glaze
Melt the butter in a medium saucepan over medium heat. Continue to cook and swirl your pan to lightly brown the butter. Once the butter has started to foam and is a light golden color, add the chopped strawberries and rhubarb. Cook until soft, about 5 minutes, stirring occasionally.
Puree the fruit mixture with the milk in a food processor or blender. Stir in the vanilla extract then let sit until the mixture has cooled.
Once cooled, whisk in the powdered sugar until smooth. Dip the donuts in the glaze then let them sit on the cooling rack or plate for a couple minutes.
Notes
The glaze won't completely set because it is more of a syrup. If you want the glaze to set, you can melt 2 tablespoons of white chocolate chips into the pureed fruit mixture before letting cool. I made it both ways and thought it tasted good without the white chocolate so I didn't feel the need to add them just to make the glaze set more.
Recipe by Lemon & Mocha at http://www.lemonandmocha.com/2018/06/08/beet-donuts-with-a-brown-butter-strawberry-rhubarb-glaze/