Combine the flour, potato starch, sugar, baking soda and salt in a mixing bowl. Then grate the frozen butter into the mixing bowl. Using your hands rub the butter into the flour mix. Because the butter is frozen, it will not completely combine with the flour. This will help make the cookies light and fluffy when you bake them. Remember to think of the cake like texture of a New York black & white cookie.
Whisk together the egg, yoghurt, vanilla, lemon juice + rind.
Add the yoghurt mixture to the flour/butter mixture and mix gently until just combined. Don't over mix as you will make the cookies too dense. Finally add the blueberries, stir with a rubber spatula until they are mixed through. The cookie batter will be very wet, it'll be much closer to cake batter than cookie dough.
Spoon onto a baking tray and bake for 12-14 minutes depending on the size of cookies you've spooned out.