Preheat oven to 400F. Grease a 12 standard muffin tin. Whisk flour, sugar, cornstarch, baking powder, salt and nutmeg in a bowl. In a separate bowl, whisk together buttermilk, melted butter, eggs and yolk. Add wet ingredients to dry ingredients and stir with rubber spatula until just combined. Scoop batter into prepared tin. Bake until muffins are lightly browned and toothpick inserted in centre comes out clean (19-22 minutes). Let muffins cool in tin for 5 minutes while you make the topping. Whisk sugar and cinnamon together in a bowl. In a microwavable bowl, melt butter in the microwave. Remove doughnut muffins from tin. Working with one at a time, brush all over with melted butter, then roll in cinnamon sugar, pressing lightly to adhere. Transfer to wire rack and let cool for 15 minutes. Serve.