Step 1

In a small saucepan over medium, bring the broth, covered, to a simmer. Reduce to low to keep warm. In a small bowl or measuring cup, combine ½ cup of the hot broth and the saffron. Set aside.

Step 2

In a large saucepan over medium-high, melt 2 tablespoons of butter. Add the rice and cook, stirring constantly, until translucent at the edges, 1 to 2 minutes.Add 2½ cups of the remaining hot broth and bring to a boil, then reduce to medium and cook, stirring frequently and briskly, until the grains are almost tender but still quite firm at the core (it will be quite soupy), 8 to 10 minutes; adjust the heat as needed to maintain a vigorous simmer.

Step 3

Add the saffron broth and cook, stirring frequently and briskly, until the rice is just shy of al dente but still soupy, 3 to 5 minutes. If the rice is thick and dry but the grains are still too firm, add the remaining hot broth in ¼-cup increments and continue to cook, stirring, until the rice is just shy of al dente.

Step 4

Off heat, stir in the Parmesan, ½ teaspoon salt and the remaining 4 tablespoons butter, 1 piece at a time. Taste and season with salt, then stir in the vinegar. Serve immediately.