In a small saucepan, combine chopped fresh rhubarb, 3 tablespoons of sugar, and enough water to cover. Cook over medium heat for about 10-12 minutes until soft. Blend until smooth to create rhubarb puree.

Preheat your oven to 350°F (175°C). Grease a standard donut pan with cooking spray or oil.

In a large bowl, whisk together all-purpose flour, light brown sugar, baking soda, baking powder, and salt until no lumps remain.

In another bowl, mix egg, buttermilk, vegetable oil, rhubarb puree, vanilla extract, and orange zest until smooth.

Fold the wet ingredients into the dry ingredients gently until just combined; it's okay if slightly lumpy.

Transfer the batter to a ziptop bag, snip off a corner, and pipe into the donut pan, filling each mold about ¾ full.

Bake in the preheated oven for about 12 minutes until they spring back lightly and a toothpick inserted comes out clean. Cool in the pan for 10 minutes before transferring to a wire rack.

Whisk together powdered sugar, rhubarb puree, and whole milk for the glaze until smooth. Add a few drops of red food coloring if desired.

Once cooled, dip the tops of the donuts into the glaze, allowing excess to drip off. Garnish with additional orange zest.