Preheat oven to gas mark 4/350oF/180oC and lightly grease a doughnut pan then set aside. In a microwavable bowl melt the butter in the microwave at 30 second increments and also set aside.
To a medium sized bowl add and combine the plain flour, salt, baking powder, cinnamon and nutmeg. Then to a second bowl add and combine the milk, sour cream, sugar, egg and vanilla extract.
Pour the wet ingredients into the dry ingredients and combine but be careful not to overmix. Using a spoon or piping bag add the batter to doughnut pans and bake in the middle of the oven for 15 minutes.
Remove from the oven and allow to cool in the pans for 5 minutes then remove the doughnuts from the pans and transfer to a cooling rack while you make the topping.
Melt the butter in a microwavable bowl at 30 second increments. In a separate small bowl, mix together the granulated sugar and ground cinnamon. Take a doughnut, lightly dip it in the melted butter coating both sides then dip it into the sugar and cinnamon also coating both sides. Place the doughnut back on the rack then repeat the process until all the doughnuts are coated.