Sweet Potato Cardamom Buns with Pistachio and Orange

Name: Sweet Potato Cardamom Buns with Pistachio and Orange
Ingredients: 2/3 cup whole milk; 8 tablespoons (1 stick) unsalted butter, cut into small pieces, plus more for greasing the bowl and baking dish; 1/2 cup cooked sweet potato flesh (from 1 small roasted and/or microwaved sweet potato); 1 large egg, beaten; 3 to 3 1/4 cups all-purpose flour, plus more for work surface; 1/4 cup packed dark brown sugar; 1 (1/4-ounce) packet active dry yeast (about 2 1/4 teaspoons); 1 tablespoon finely grated orange zest; 1 teaspoon ground cardamom; 1/2 teaspoon kosher salt