Preheat your oven to 160C Fan / 180C.
Grease a cupcake / muffin tin either with butter or spay a little oil and spread it around each hole.
Place your gluten free flour, baking powder, xanthan gum, cinnamon and caster sugar into a large bowl and gently mix.
In a small bowl, mix together your melted butter (must be cooled), oil, buttermilk and egg. Whisk until combined.
Pour your wet ingredients into your bowl of dry ingredients and mix together. Try to mix as minimally as possible, just until combined. Your mixture is ready!
Spoon your mixture into the muffin holes (already greased). Fill them up to about half way. You can fill them higher but they will just look a little more muffin like in shape - still delicious!
Place in the oven for 16-18 minutes and cooked through. Once cooked allow to cool for a few minutes in their tin before gently easing them out.
Mix your granulated sugar and cinnamon together. Whilst still warm carefully coat each in melted butter and then roll or sprinkle each with cinnamon sugar. Allow to cool.
Making sure your apple filling is cool, thick and smooth, spoon it into a piping bag with a medium nozzle on the end.
Then from the side of your doughnut, carefully push your nozzle into the doughnut and squeeze in your apple filling until lovely and full. Enjoy!