Add frozen raspberries to a pot and heat over medium heat until the berries start braking down, usually about 7-10 minutes
Bring rasperries to a boil. Add sugar and stir. Bring back to a boil.
In a seperate small bowl, whisk together the corn starch and water until all the lumps are smooth. Pour into the raspberry mixture, stiring vigorously for about a minute or two. The mixture will thicken and you can remove from the heat.
Place mixture in a seperate bowl and cover with saran wrap (you DO NOT want the raspberry filling to get a crust of coating on the top). Set aside or place in fridge to cool while donut dough is rising.
After you have made your donuts (use the air fryer yeast donut recipe), allow to cool completely after air frying. Once cooled, use a skewer to polk a hole into the side of the donut and wiggle around a bit to enlarge the whole and make space for the filling. Do this to all of the donuts.
Add your piping tip into the piping bag and then fill bag with your raspberry filling. Insert the piping tip into the hole that you created with a skewer and begin filling your donuts. You will gently start squeezing the filling into the donut and while squeezing start to pull the tip out of the donut as you are filling it so that the raspberry filling can get into as many cracks and spaces as possible! :) Fill ALL of the donuts and then set aside.
Add the granulated sugar, zest from a whole lemon and (optional) 3-7 drops of lemon essential oil in a bowl. Mix together with a fork until well combined.
In a seperate bowl, melt the butter and set aside.
Begin dipping the tops of the donuts. First lightly brush the melted butter onto the donuts (you can also dunk the tops of the donuts into the sugar, but you may need more than a few tbs of melted butter as this "sucks" up more butter). Then immediately dip the donuts into the lemon sugar topping! Set aside to cool....then eat, share with friends, and impress your neighbors! :)