Preheat oven to 400 degrees. Make sure cast iron skillet is well seasoned or grease with butter and set aside.
In a medium sized pot, combine blueberries, sugar and lemon juice over medium heat until boiling. Reduce heat and let simmer for 7-9 minutes.
Whisk together cornstarch or arrowroot powder with water until smooth. Then pour into pot with blueberries and stir until dissolved. Remove heat and let cool and thicken.
Make biscuits by first whisking together flour, baking powder, sugar, cream of tartar, salt and baking soda.
Dice cold butter straight from fridge into small cubes. Then cut butter into flour mixture. You can do this by using a fork and whisking butter into flour slightly pressing and cutting against the side of the bowl. Alternatively, you can combine butter with flour mixture in a food processor. Pulse together until crumbly.
Next slowly pour buttermilk into mixture while continuously mixing until soft, lumpy dough forms.
Pour blueberry filling into skillet. Then top with balls of biscuit dough. There should be some filling showing through between dough balls. Be careful not to over handle dough or the biscuits will be too dense.
Bake for 35-40 minutes until biscuit dough is cooked through and golden brown on top.
Serve while warm. Possibly with ice cream!