Step 1: In a large saucepan over medium heat, combine 1 cup extra-fine granulated sugar with ¼ cup water. Bring to a boil until temperature reaches 245º F.
Step 2: While sugar mixture is boiling, whip egg whites in an electric mixer until stiff peaks form. Whip in ¼ cup extra-fine sugar.
Step 3: When syrup reaches correct temperature, slowly pour into meringue in a steady stream with mixer on high. Continue beating until the outside of the bowl reaches room temperature and mixture is thick and glossy, about 10-12 minutes.
Step 4: Beat in butter pieces, one tablespoon at a time.
Step 5: Mix in rum, vanilla and cinnamon.