Preheat oven to gas mark 4/350oF/180oC/160oC fan 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. To a second bowl, add and combine both sugars, milk, sour cream, egg and vanilla extract. Add in the melted butter set aside earlier and combine well.
Pour the wet ingredients into the dry ingredients and combine but be careful not to over mix. Using a spoon or piping bag add the batter to the doughnut pan and bake in the middle of the oven for 15 minutes.
Remove from the oven and allow to cool in the pan for 10 minutes then remove the doughnuts from the pan and transfer to a cooling rack while you make the topping.
Top the doughnuts: 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.