Preheat oven to 375F/190C and stir together the flour, baking powder, salt, baking soda, cinnamon and nutmeg in a medium-size bowl.
In a separate smaller bowl, mix together the melted butter, egg, sugar, buttermilk, apple sauce and grated apple.
Pour the wet ingredients into the dry and gently stir until the mixture just comes together - don't over-mix!
Scoop the mixture into a piping bag and pipe into the six moulds of a greased donut pan.
Place in the preheated oven and bake for around 10 minutes, until a toothpick, when inserted into the centre, comes out clean. Leave the donuts in the pan for 5 minutes before tipping them out.
Whilst the donuts are cooling, stir the sugar and cinnamon together in a shallow bowl.
Brush each of the warm donuts with the extra apple sauce and dip each one in the cinnamon-sugar mixture. Serve straight away or store in the fridge inside an air-tight container for up to five days.