In a large bowl, whisk together the gluten free flour, salt, baking powder, and baking soda.
Use a pastry cutter to cut the butter into the flour mixture until it resembles coarse cornmeal.
Add the buttermilk and stir in the buttermilk until incorporated. Knead the dough in the bowl to bring it together into a ball.
Turn the dough onto a floured surface and pat it into a rectangle.
Fold the rectangle into 3rds (like a letter) and turn the dough a half turn.
Then flatten into a rectangle until the dough is about a ½-¾ inch thick. Repeat the laminating process 3 times. After the last fold, form the dough into a square, about 1 ½ inches thick.
Use a large knife or bench scraper to cut the dough into 8 even pieces.
Place the biscuits close together on the prepared baking sheet with about 1 inch between each biscuit.
In a small bowl, mix together the hot water and baking soda to make the baking soda wash. Mix until the baking soda is mostly dissolved.
Liberally brush the biscuits with the baking soda wash. It’s ok if some of the wash is leftover.
Place the sheet pan with the biscuits in the freezer, uncovered, for 30 minutes. Preheat oven to 425ºF.
Once the biscuits have chilled, brush the top of each biscuit with the egg wash and sprinkle with coarse salt.
Bake for 14-16 minutes or until done.
Serve immediately or at room temperature.
