Mix sugar (1 tbsp), salt and dry yeast in a bowl. Pour in half of the warm milk (100F) and add 2 tbsp of all purpose flour, beat with a whisk. Cover the bowl with cling film and leave for 15 minutes.
Mix the remaining dry ingredients together in a bowl (remaining flour, sugar). Add to the bowl after 15 minutes or when foam appears on top of yeast. Add vanilla bean paste and egg after that.
Knead soft and slightly sticky dough. Don't add too much flour as the donuts will be dense and heavy. Cover the bowl with cling film and leave for 40-60 minutes in a warm place. After an hour, the dough should have doubled in size.
On a non stick cutting surface roll out the dough to 1 inch thick and cut out round portions about 2 inches in diameter with a cookie cutter or top of a glass. You will get about 8-10 donuts in total depending on the size. Cover them with cling film and leave for 20 minutes.
Preheat the air fryer to 380 F. Spray the basket with olive oil and place the donuts in the basket. Air fry for 4 minutes on one side. After 4 minutes, flip the donuts to the other side and fry for another 3-4 minutes or until golden brown and no longer sticky inside.
** If frying in oil heat to 180C/350F before slowly adding dough until browned on each side, a few minutes on each side. Set on paper towels when done to absorb oil.
Transfer the lemon curd to a piping bag fitted with a round or open star tip. Let the donut cool slightly, then make a hole in the side and fill it with lemon curd. Sprinkle them with powdered sugar and serve.