Place the flour, sugar, salt and nutmeg in a large bowl of a standing mixer and stir to combine. Add an egg, milk, 40 g of melted and cooled butter and the yeast. With the mixer running on low, bring the ingredients together, until they start to form a dough. Increase the speed to medium, and mix until the dough no longer sticks to the sides of the bowl, about 3-5 minutes. You can also knead the dough by hand, but it will take longer. If kneading by hand, transfer the dough into a clean work surface and knead for about 10- 15 minutes, until it is smooth and elastic.
Transfer the dough into a large, lightly greased bowl. Cover and let it rest until it has doubled in size, about 2 hours.
Dust the work surface with flour, then transfer the dough onto it. Using the rolling pin, roll the dough to about 1 inch thickness. Make sure not to roll the dough too thin, as it will make the doughnuts flat. Using the doughnut cutter or pastry cutting rings (medium and small one), cut the doughnuts out from the dough. Re-roll any leftover dough and repeat until you have used all the dough. Transfer the doughnuts into a large baking sheet lined with some parchment paper, brush with the additional 40 g of melted butter, and leave them to rest for 45 minutes.
When ready, preheat an air fryer to 180°C. Lightly grease the basket/ heated plate and gently place the doughnuts inside (fry 2-4 doughnuts at a time, depending on the size of your basket). Fry for 4 minutes, or until golden, then remove from an air fryer.
Combine the sugar and cinnamon in a large bowl. Brush the still warm doughnuts with any remaining melted butter, then toss them in cinnamon sugar. Enjoy!