Preheat oven to 400 degrees. Butter a four cup casserole dish and sprinkle with sugar.
In a metal bowl, melt the chocolate chips over a saucepan over simmering water. Stir the chocolate until it is smooth. Remove the bowl from the saucepan and whisk in the egg yolks one at a time. Whisk in the liquor and melted butter.
In a large bowl, Using an electric mixer beat the egg whites with a pinch of cream of tartar until they are stiff and form a peak when the beaters are lifted up from the whites. Stir 1/4 of the stiff egg whites into the chocolate mixture and mix until smooth. Using a rubber spatula, carefully fold the remaining egg whites into the chocolate mixture until just combined. Pour the mixture into the prepared casserole dish and bake for 12 minutes or until puffed. Serve immediately