Pour the stock into a large pot and bring to a simmer over medium-high heat Reduce the heat so the broth simmers gently, add the tortellini and cook until they float to the surface, about 2 minutes. While the tortellini are cooking, put 1 tablespoon of butter in the bottom of each of 4 to 6 bowls. Ladle the tortellini and broth into the bowls; the hot broth will melt the butter. Grate the cheese over each serving and serve immediately.