Preheat the oven to 375 degrees F. Butter and sugar 6 (6-ounce) ramekins. Heat the butter, 1 tablespoon sugar, hazelnut liqueur, and vanilla in a double boiler over medium heat until the butter melts. Remove the butter mixture from the heat, add the chocolate, and let sit until it melts, about 3 minutes. Place the chocolate mixture in a pie dish and place in the freezer for 10 minutes to firm up. Use a spoon to form the chilled mixture into 6 evenly-sized balls about the size of a walnut. Reserve in the refrigerator.
Meanwhile, place the flour in a double boiler and slowly whisk in the milk. Add the salt. Heat the mixture over medium heat, whisking constantly until thick, about 5 minutes. Add the egg yolks and continue to whisk constantly. The mixture will thicken to the consistency of mayonnaise in another 3 to 4 minutes. Remove the mixture from the heat. Stir in the 6 ounces of chocolate and set aside to let the chocolate melt. Place the egg whites in a large bowl with the cream of tartar. Using a hand mixer, whip the egg whites until soft peaks. Gradually add in the 1/4 cup sugar and continue whipping until firm peaks. Fold the egg whites into the warm chocolate mixture.
Place a ball of the chilled chocolate mixture in each of the ramekins. Spoon the souffle mixture over the chocolate balls and up to the rim of the ramekins. (At this point the souffle can be covered and kept refrigerated for 2 days.) Place the ramekins in a hot water bath and bake until golden on top and the souffle has risen, about 30 minutes (40 minutes if refrigerated). Remove from the oven and serve immediately.