In a bowl, stir sugar, milk, and dry yeast. Allow yeast to activate for 2 - 5 minutes.
Pour all your dry ingredients: flour, sugar, and salt into a large bowl, then stir.
Add your wet ingredients (milky yeast, melted butter, and whisked egg) to the dry ingredients.
Use your hands to combine them all until they form a ball. Lightly flour your kitchen countertop, knead your dough by hand for 5 - 7 minutes or use a dough hook in an electric mixer for 3 - 4 minutes. Press your dough into a ball. Then, in a warm place, put the dough in a clean bowl and cover it. Allow it to rise for 30 minutes.
Roll out the dough so that you can cut approximately 12 donuts.
Leave them on the baking sheet.
Let them rise on a baking sheet for 30 - 60 minutes.
Use the cooking oil spray to grease your air fryer’s compartment lightly. Place the donuts in the air fryer without overlapping the dough.Once the air fryer is on, choose the frying setting, set the temperature to 340F (170C), and the time for 4 - 5 minutes. Then cook the donuts. While cooking the donuts, stir the caster sugar and cinnamon in a bowl.
When the air fryer finishes cooking, keep your donuts in the compartments and spray them with cooking oil. Next, remove them from the compartment and coat them with the sugar mixture.
Serve warm!