In a pitcher, mix together the whiskey, syrup, sherry, and bitters if using. Chill thoroughly.
Before serving, fill small highball glasses (or half-pint jelly jars) with snow, packing it down lightly. Drizzle in the cold whiskey mixture, adding as much or as little as desired. Serve with a small spoon.
"The New Midwestern Table" by Amy Thielen (c) Clarkson Potter, 2013. Provided courtesy of Amy Thielen. All rights reserved.