Place the egg whites in the bowl of a stand mixer fitted with whisk attachment, then whisk until soft peaks form.
In a separate bowl, whisk together egg yolks with cold seltzer, schmaltz, chopped parsley, chopped dill, Kosher salt, and of black pepper.
Fold the egg whites into the egg yolk mixture until just combined. Add matzo meal, then stir gently until thoroughly mixed. Cover with plastic wrap, then refrigerate.
Place water in a large soup pot with 1 bouillon cube, then bring to a gentle boil. Scoop about ¼ cup of batter at a time , then with slightly wet hands, roll gently into a ball.
Cover with a lid and cook. While it's tempting, do not remove the lid to check on the matzo balls before the timer goes off.
Transfer 1 or 2 matzo balls to each serving bowl, then serve with your favorite chicken soup. Garnish with additional parsley or dill.