In a 6-quart saucepan, melt butter over medium. Add vermicelli and cook, stirring frequently, until golden brown, about 4 minutes. Stir in rice and cook until translucent, 2 minutes.

Add stock and 2 1/2 teaspoons salt and bring mixture to a boil. Reduce heat to low, cover, and simmer until all liquid is absorbed, 25 to 30 minutes. Remove from heat and let stand 5 minutes. Fluff with a fork before serving.