Cube the butter and pop it in the freezer for 10-15 minutes (you want it to be solid but not rock solid, so go by howcold the butter was when you sliced it). Combine the dry ingredients in a medium-large mixing bowl.
Rub the butter into the flour using your fingertips. You want fine small pieces of butter distributed throughout the dough – they will create lift in the oven. Make sure the chunks are all small - they should look like a fine meal.
Pour the yoghurt over the flour and use a spoon to combine. Add the milk as necessary to form a dough that looks dry but feels wet. There should be no dry spots flour on the bottom of the bowl, but the dough should only just hold together. Add the blueberries and gently use your hands to distribute them. Once all the flour has been picked up off the bowl and the mixture is combined, cover it and place in the fridge for 30 minutes.
Flour a clean, dry surface with tapioca flour. Sprinkle some flour onto the dough, then tip it out onto the bench. Flour any sticky surfaces of the dough, then use your hands to form it into a rectangle shape. Slice the rectangle into quarters, then stack each piece on top of the first until you have a little tower. Use a floured rolling pin to gently flatten the dough into a rectangle about 20cm long and 5-6cm thick. A tall dough will create taller scone, and smooth, flat dough surface will create flat scone tops.
Create 5-6 large scones. I find it far easier to simply cut these scones with a sharp knife (in slices or squares) than to use a cutter. They don’t grow much in the oven, so they should be nice and tall as you cut them. Don’t twist the cutter as you dip it into the dough – this has the potential to tear up those chunks of butter we made. Gently place the scones in a lined cake tin. I use a 20cm one. Keeping the scones snug will encourage them to grow upward instead of spread. Place them back into the fridge for at least 30 minutes or longer if you have the time. You can also put them in the freezer for 15 minutes.
When you’re ready to bake, preheat the oven to 200C. Brush the scones with the egg wash, then place them in the oven for 25-30 minutes, or until they are cooked through and the tops are golden. Serve with jam and lactose free whipped cream, or whatever else you’re into. Microwave or steam bake leftover scones to restore them to their former glory.