Coat a large straight sided porcelain baking dish or souffle mold (about 8-inches diameter and 4- deep) with the softened butter. Alternatively, you can use eight individual straightsided porcelain ramekins. Coat evenly and generously all the way to the top, making sure to get the corners. Sprinkle in some sugar and toss around to coat the bottom and sides of the mold or molds. Place molds in the refrigerator to chill (if using individual molds, place them on a cookie sheet first.)
Preheat oven to 400 degrees. Find a medium metal bowl that completely covers the top of a small saucepan. Place 3 inches of water in the saucepan and bring to the simmer. Turn off the heat. Place cut up butter and chocolate in the bowl and place bowl over pan of hot water. Let sit for 2 minutes then stir. It should be completely melted; if not, remove the bowl, bring the water back to a simmer, turn off the heat and replace the bowl. Stir until smooth. Stir in the Grand Marnier. Set aside to let cool to room temperature. Place egg yolks in the bowl of an electric mixer fitted with the whip. Beat egg yolks at medium speed until opaque and lightened in color, about 3 minutes. Add to cooled chocolate mixture.
Bring medium pot of water to a boil. Clean mixer bowl and whip well, and dry them. Place egg whites in bowl and whip at high speed 2 minutes, then reduce speed to medium and add the cream of tartar. Whip until soft peaks form then slowly add the granulated sugar down the side of the bowl while continuing to whip until firm peaks form. Take 1/3 of the whites and whisk into chocolate mixture to lighten it. Fold in remaining whites with a large rubber spatula, trying not to deflate the whites. Fold gently but thoroughly until no white streaks remain. Remove coated molds from the refrigerator. Fill mold or molds with souffle mixture, up to 1/2-inch from rim. Run your thumb around the inside of each mold to form a shallow groove all around. When all molds have been filled transfer to the oven and bake until risen and top is evenly dried, about 10 minutes for individual soufflJs and about 25 minutes for a large. Serve immediately.
Classic souffle technique: egg yolks combined with flavoring base, lightened with whipped egg whites and baked until risen. Very important that all equipment used for beating whites be scrupulously clean. Running thumb around mold encourages an even rise.