Toss the raspberries with 2 tablespoons of the sugar and the raspberry liqueur if desired. Let sit for 10 minutes.
Beat the eggs in a large bowl. Whisk in the salt and 1/2 cup of the sugar, until just blended. Sift the flour into the bowl, whisking it into the mixture just blended.
Butter 6 individual gratin dishes. Divide the raspberries among them. Divide the egg mixture among the dishes. Sprinkle with the remaining 2 tbs. 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.