Add in very cold butter into the flour mixture. Either cut in or use hands.
Add in berries. Mix gently.
Add baking soda to buttermilk.
Add buttermilk to mixture. Mix with wooden spoon, adding in a little more flour (no more than 1/4 cup) r as needed so the mixture isn't too wet. It will be wet to a degree, but must be dry enough to cut into ten medium sized biscuits. Place biscuits in cast iron pan or baking sheet with sides touching.
Brush biscuits with a little milk and sprinkle with sugar.