01 - Bring a large pot of salted water to a rolling boil. Add the tagliatelle or fettuccine and cook according to package directions until al dente. Reserve 0.5 cup of the pasta water before draining. Set the drained pasta aside.
02 - In a large skillet or sauté pan over medium heat, melt the unsalted butter. Pour in the heavy cream and bring to a gentle simmer.
03 - Stir in the cooled brewed espresso, granulated sugar, and optional instant espresso powder. Allow the mixture to simmer for 2 to 3 minutes, until it begins to slightly thicken.
04 - Reduce the heat to low. Whisk in the mascarpone cheese, vanilla extract, and a pinch of salt until the sauce is smooth and creamy.
05 - If using, stir the coffee liqueur into the sauce. Taste and adjust sweetness as necessary.
06 - Add the cooked pasta to the skillet with the sauce. Gently toss to ensure the pasta is evenly coated. If the sauce appears too thick, add the reserved pasta water, one tablespoon at a time, until the desired consistency is achieved.
07 - Divide the pasta among serving plates. Generously dust with unsweetened cocoa powder and sprinkle with shaved or grated dark chocolate. Add fresh mint leaves for garnish, if desired.
08 - Serve immediately, allowing guests to savor the warm, aromatic flavors reminiscent of tiramisu.