In 2-quart saucepan, sprinkle gelatin on cold water to soften; stir in 1 cup granulated sugar. Heat to a rolling boil; reduce heat. Simmer uncovered 10 minutes. In large bowl, place powdered sugar and hot gelatin mixture. Beat with electric mixer on medium speed about 2 minutes or until foamy. Add vanilla, baking powder and salt; beat on high speed 12 to 15 minutes or until soft peaks form. Frost bottoms of cookies with frosting. Let stand until frosting is dry, 2 to 3 hours, before storing.