- Preheat the oven to 400F and line a large baking sheet with parchment paper and set aside. 
- In a large bowl whisk together flour, salt, baking powder and baking soda. 
- Cut frozen or very cold butter into ½ inch cubes and add to the flour mixture. 
- Working quickly, smash the butter into the dough using your hands, pastry cutter or two forks until peas-sized balls have formed. 
- Slowly stream the buttermilk into the flour mixture. 
- Using a fork, stir together the flour and buttermilk mixture until a shaggy dough forms. 
- Turn the dough out onto a lightly floured work surface and gently knead until the dough just comes together, and shape it into a rough rectangle. 
- Gently roll the dough into a rectangular slab that is ½ an inch thick. 
- Carefully fold the dough in thirds, so it overlaps like a letter or trifold. 
- Roll the dough into a rectangle that is roughly ¾ inch to 1 inch thick. 
- Lightly dust the sharp side of a 3 or 4 inch round cookie cutter and punch out as many biscuits as possible and transfer to the prepared baking sheet. 
- Reroll the scraps and punch out more biscuits until the scraps can’t be rerolled and press the remaining scraps into a free-form biscuit. 
- Place the tray in the freezer for 10-15 minutes to firm up the butter. 
- Brush the biscuit tops with buttermilk. 
- Place the tray in the middle rack of the oven and bake for 15-20 minutes until the biscuits are golden brown. 
- While the biscuits are baking prepare the gravy. 
- In a skillet set over medium heat, melt 2 tablespoons of butter and saute the mushrooms until their liquid has cooked off and they’re browned. 
- Add in the rosemary and onion powder and stir for one minute, then season to taste with salt and pepper. 
- Add two tablespoons of butter to the pan and once it’s melted sprinkle flour over everything and cook for about 1 minute or until a blonde roux forms. 
- Whisking constantly, add milk to the pan mixture in ¼  cup splashes at time, making sure everything is smooth before adding in the next splash. 
- Repeat until you’ve used up all the milk, then adjust the heat to medium-low and simmer for 3-4 minutes until everything thickens to a gravy consistency. 
- Adjust salt and pepper to taste. 
- Ladle the gravy over the fresh biscuits and enjoy while hot!