Place egg whites in a large bowl; let stand at room temperature 30 minutes. Preheat oven to 225°., Place sugar and strawberries in a food processor; process until powdery. Add gelatin; pulse to blend., Beat egg whites on medium speed until foamy, adding vanilla if desired. Gradually add gelatin mixture, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved. Continue beating until stiff glossy peaks form., Cut a small hole in the tip of a pastry bag; insert a #1M star tip. Transfer meringue to bag. Pipe 2-in. roses 1-1/2 in. apart onto parchment-lined baking sheets., Bake until set and dry, 40-45 minutes. Turn off oven (do not open oven door); leave meringues in oven 1-1/2 hours. Remove from oven; cool completely on baking sheets., Remove meringues from paper. Dip bottoms in melted chocolate; allow excess to drip off. Place on waxed paper; let stand until set, about 45 minutes. Store in an airtight container at room temperature.