Biscuit Doughnuts with Lemon Cream Filling and Blueberry Sauce

Name: Biscuit Doughnuts with Lemon Cream Filling and Blueberry Sauce
Ingredients: 1/2 cup mascarpone cheese, at room temperature; 1/2 cup lemon curd; Grated zest of 1 lemon, plus 1 teaspoon fresh lemon juice; 1 pint blueberries; 1/4 cup granulated sugar; 2 teaspoons fresh lemon juice; 6 cups canola oil; 4 cups all-purpose flour, plus for dusting; 4 teaspoons baking powder; 1 teaspoon baking soda; 1 teaspoon salt; 12 tablespoons (1 1/2 sticks) cold unsalted butter, cut into small pieces; 1 1/2 cups cold buttermilk; Confectioners' sugar, for rolling