1. Preheat the oven to 375 degrees F.
2. Make the mushroom broth as per requirement.
3. In a large bowl, grate the frozen butter, Add the flour, baking soda and baking powder to it and mix well with hand to bread crumbs consistency till well combined.
4. Pour the buttermilk into it and fold in the mixture to form loose dough. Do not knead it.
5. Sprinkle flour on a flat surface and roll the dough on it.
6. Dip a cookie cutter into flour and cut the cookies from the dough.
7. Place them carefully on the baking tray and bake for 15 minutes or till done. Check it after 10 minutes.
8. Put the leeks into the mushroom gravy and let it simmer till it thickens to required consistency.
9. On a serving plate, place the biscuit. Cut it into half and pour the mushroom broth all over it. Place one half over the other to form a burger. Garnish with truffle butter and serve hot.