Preheat the oven to 400 degrees F. Butter 4 individual ramekins. Sprinkle each ramekin with 1 teaspoon sugar. In a large metal bowl, set over a pot of simmering water, melt the chocolate, whisking it occasionally. Remove the bowl from the heat. In a mixing bowl, whisk the egg whites with 1/4 cup of the sugar until stiff and glossy. Whisk the egg yolks into the chocolate one at a time, add the Grand Marnier, and whisk in the remaining sugar. Fold in the egg whites and blend until smooth. Pour the chocolate mixture into the prepared ramekins. Place the ramekins on a baking sheet and bake until they are puffed and somewhat firm, about 20 to 25 minutes. Remove from the oven and serve with the chocolate sauce and powdered sugar.