Begin by mincing the shallot and very thinly slicing the mushrooms.
In a small sauce pot, add the white wine and the white wine vinegar along with the minced shallot and whole peppercorns.
Bring this to a gentle simmer.
Once the liquid has reduced by about half, add the sliced mushrooms.
Continue to simmer until the contents of the pot are au sec, or almost dry, with a small amount of liquid remaining.
Pour all of the contents of the pot into a fine mesh strainer placed over a bowl.
Use the back of a spoon or spatula to gently press all liquid out of the mushrooms and shallots. There will be a very small amount of brown liquid collected. Probably no more than 1 tablespoon.
Add the brown liquid back to the pot, off the heat.
Slice the cold butter into 1 tablespoon sections, 8 in total.
Off the heat, add 1 tablespoon of cold butter to the liquid, using a rubber spatula to stir continuously as it melts. The butter should emulsify into the sauce without separating.
Repeat this step with each pat of butter, adding no more than 2 at a time and not adding the next one until the previous has fully melted/emulsified.
Once all the butter has been added, add the heavy cream and truffle sauce and stir to combine. Season with salt and taste to adjust to your liking. If the pot is too cold, let it heat over the lowest flame possible, too much heat will cause the sauce to separate.
In a large pot of salted water at a rolling boil, add the fresh tagliatelle.
Stir immediately once added to avoid any sticking together. Cook until the pasta begins to float (if fresh. If using dry pasta, cook according to box instructions).
Strain pasta and add directly to the truffle sauce while still hot.
Toss to combine, taste, adjust seasonings to your liking.
Serve hot and enjoy! Optional: Garnish with fresh shaved truffles or chopped chives.