Preheat oven to 375 degrees.
Toss the raspberries with 2 tablespoons of the sugar and the raspberry liqueur if desired. Let sit for 10 minutes.
Butter 6 individual gratin dishes. Divide the raspberries among them. Divide the egg mixture among the dishes. Sprinkle with the remaining 2 tablespoons sugar. Place on cookie sheet in the oven. Cook for 15 minutes, or until the top has just begun to harden and the center is still creamy. Pass under the broiler for a moment to brown. Serve immediately.
Suggested wines: New York Finger Lakes Riesling; Sauternes; and/or Muscat de Beaumes-de-Venise