Buttermilk Doughnuts

INGREDIENTS

• :
• 3½ cups all-purpose flour
• ¾ cup granulated sugar
• ½ teaspoon baking soda
• 2 teaspoons baking powder
• 1 teaspoon salt
• 1 teaspoon freshly grated nutmeg
• 1 teaspoon ground cinnamon
• 2 eggs
• ¾ cup buttermilk
• ¼ cup sour cream
• ¼ cup unsalted butter, melted and cooled
• Vegetable oil for frying
• 4 ounces dark chocolate, coarsely chopped
• ½ cup heavy cream
• 2 tablespoons unsalted butter
• 2 cups powdered sugar
• ¼ cup whole milk
• 1½ teaspoons vanilla extract
• 1¼ cups granulated sugar
• 3 tablespoons ground cinnamon