Mix together the vanilla extract, yogurt, flour, sweetener, baking powder and egg in a mixing bowl and knead until you have a lovely big dough
Divide and shape into 12-15 balls, depending on size preference
Air-fry at 200°C for about 10-12 minutes
Once cool enough to handle, make a hole in each doughnut
Fill a piping bag with the jam and pipe some jam into each doughnut (or skip this step and just dip them into the jam instead)
Melt the butter in the microwave for 1 minute on high
Roll the balls first into the melted butter, then into the caster sugar to coat
Enjoy
