Rehydrate the yellowfoot mushrooms in the stock and wine for 30 minutes until soft, then agitate them to remove any dirt. Remove the mushrooms, strain the liquid, then recombine the two.
Heat the mushrooms and liquid until it is reduced by 75%, then season to taste with salt and reserve.
Heat the mushroom sauce and whisk in 1 tsbp of unsalted butter until the sauce thickens slightly. Keep the sauce warm and reserve until ready to plate. You may need to whisk the sauce occasionally to prevent it breaking. If the sauce starts to break and look oily, add a tbsp cold water, return it to the heat, and whisk vigorously to re-emulsify it.