Chop rhubarb into one-inch chunks and add to a medium saucepan with 4 cups of water. Bring to a boil, then cover and simmer for 15 minutes.

Strain liquid through a fine mesh strainer, pressing solids to extract juice. Set aside.

Pour the juice into a jar, leaving solids behind.

Combine 1/2 cup rhubarb juice, lemon juice, eggs, yolk, sugar, and salt in a stainless steel bowl over simmering water. Whisk constantly until thickened and reaches 180?F, about 10 minutes.

Remove from heat and let cool, occasionally stirring. Strain into a blender and gradually add butter until fully incorporated.

For the donuts, bring milk to a strong simmer and let cool to 130?F. Sprinkle yeast over and stir. Set aside.

In a stand mixer, beat flour, sugar, and kosher salt to combine. Add milk mixture and eggs. Mix on low, then knead with a dough hook for about 3 minutes.

Gradually add butter until dough is smooth and cohesive. Transfer to a greased bowl and cover with plastic wrap.

For same-day donuts, let proof for 1.5 hours or until doubled. For overnight donuts, refrigerate to proof slowly.

Line two baking sheets with parchment and spray with nonstick spray.

Once doubled, roll dough to a 9x13 rectangle about 1/2 inch thick. Cut out 12 donuts and arrange on baking sheets.

Cover with plastic wrap and let rise in a warm area for 90 minutes.

Heat 2 quarts of vegetable oil in a dutch oven to 375?F. Mix 1/2 cup sugar with 2 teaspoons ground cardamom in a bowl.

Fry donuts until golden brown, about 1-2 minutes per side. Remove and place on paper towels.

Toss fried donuts in the sugar mixture and fill with rhubarb cream using a pastry bag.