Preheat oven to 350ºF. Butter a 3-quart round soufflé or other baking dish.
In a saucepan over medium heat, melt butter. Whisk in flour until smooth. Whisking constantly, slowly pour in milk and cook until thick and bubbling, about 3 minutes. Reduce heat to low and stir in cheese until melted; remove from heat.
In a bowl, stir together egg yolks, salt and 1 tsp. paprika. Whisk in cheese sauce and rice. In another bowl, with an electric mixer on high speed, beat egg whites with cream of tartar until stiff but still glossy. Stir 1/4 of beaten egg whites into rice mixture to lighten, then gently fold in remaining egg whites. Spoon into prepared dish and dust with remaining 1/4 tsp. paprika. Bake until puffy, golden brown and just set in center, 45 to 50 minutes. Serve immediately.