In a drink shaker, combine blood orange juice and simple syrup and shake for 15 seconds.
Divide equally between 4 champagne glasses. Top with prosecco. Add little "floater" of 1/2 oz Grand Marnier each.
Stir gently to combine and garnish with fresh blood orange slices, if desired.
Notes
* blood orange juice can be made by squeezing your own fresh or purchasing pre-made** optional - to make simple syrup, heat equal parts sugar and water until sugar dissolves. Let cool completely before adding to punch.