Bring 2 1/2 cups of the broth to a rolling boil in a large pot.
Add the frozen cheese tortellini and return to a boil; cook until the tortellini float, about 3 minutes, and reach 165°F internally.
Drain the cooked tortellini well in a colander and set aside.
Meanwhile, whisk the remaining 1 1/2 cups of broth with 2 tablespoons cornstarch until smooth.
Melt 4 tablespoons butter in a large non-stick skillet over medium-high heat until foaming.
Add the minced garlic and dried basil to the skillet and cook, stirring, for about 2 minutes until fragrant but not browned.
Whisk in the broth–cornstarch mixture and bring to a boil, then simmer until the sauce thickens, about 2 minutes.
Add the drained tortellini to the skillet and toss gently to coat in the garlic butter sauce.
Transfer to a serving dish and garnish with freshly grated Parmesan and chiffonaded basil, if using.
