Prep: Rehydrate some dried sliced porcini mushrooms by placing them in a small bowl with boiling water. Cover and leave for about 30 minutes. Remove the rehydrated mushrooms to a separate bowl; strain and reserve the porcini broth left behind.(By all means, use some white truffles instead, if you happen to have some laying around)
Prep: Grate 1/4 cup, loosely packed, of good quality parmesan cheese (per serving), aged a MINIMUM of 24 months. Younger or lower quality cheese may tend to form shreds and strings, rather than melting evenly into a creamy sauce.
Prep: Prepare one to two portions -- about 1/2 tablespoon each -- of butter per serving; get this very cold, almost frozen.
Now you're ready to cook. Prep your cookware and tools. You will need: a pot of generously salted, boiling water; a small saucepan at temp over medium-low heat; a small whisk or fork; a spoon or small ladle; salt and pepper.
Drop a serving (about 60 grams) of your freshly made tagliolini to cook. This will cook quickly, so everything after this step happens quite fast.
Add a small ladle of salted pasta water, two or three slices of your rehydrated porcini mushrooms, and a spoonful of the porcini broth to your saucepan.
Mount well-chilled butter into the sauce, stirring vigorously with a whisk or a fork to emulsify. You'll have to do this by feel, but will probably end up using roughly a tablespoon of butter at this stage.
Around this time, your pasta should be cooked. Transfer it directly into your saucepan, and mix to coat the pasta evenly. Adjust with more pasta water, and mount more butter into the sauce if needed.
Remove from heat; dump in your parmesan cheese and mix until incorporated. The sauce should be creamy and glossy, but not too thick; about the consistency of raw egg yolk.
Season to taste with salt (remember that the pasta water is also salted) and pepper; plate and serve immediately.