Preheat oven to 400°; place a heavy baking sheet on middle rack.
Coat 6 (8-ounce) ramekins with cooking spray, and sprinkle each dish with 2 teaspoons granulated sugar, shaking and turning to coat.
Place egg yolks in a large bowl; beat with a mixer at medium-high speed 5 minutes or until thick and pale. Gradually add 1/4 cup granulated sugar; beat 2 minutes. Beat in liqueur and vanilla.
Place egg whites in a large bowl; beat with a mixer at high speed 1 minute or until foamy using clean, dry beaters. Add the cream of tartar and salt; beat mixture until soft peaks form. Gradually add 1/4 cup granulated sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently stir one-fourth of egg white mixture into liqueur mixture. Gently fold in the remaining egg white mixture; divide evenly among the prepared ramekins.
Place soufflé dishes on baking sheet in oven; bake at 400° for 10 minutes or until tall and golden brown (soufflés will rise 1 1/2 to 2 inches above the dish rim). Quickly dust soufflés with powdered sugar. Serve immediately.
Note: Using a wide rubber spatula, gently fold egg whites into the batter by making a scooping motion from the bottom of the bowl to the top. This prevents the egg whites from deflating.