Place the sumac berries in a 1-quart (1 L) mason jar. Pour in the Aperol, seal the jar, and let steep at room temperature for four days. Strain through a fine-mesh sieve and return the Aperol to the mason jar. The infused Aperol will keep indefinitely.
Cocktail:
Rim a wine glass with the ground sumac: pour the sumac powder onto a small plate, dip the rim of the glass in water, then dip it in the powder to coat the rim evenly.
Add the ice cubes to the glass. Pour in the infused Aperol. Add the Prosecco and soda water. The bubbles will do the mixing for you. Garnish with an orange peel.
