Line 20 cm souffle dish with plastic wrap, letting excess extend over edge; set aside. Combine yolks, quarter cup sugar, flour and arrowroot in medium saucepan. Whisk in cream, milk and kirsch. Place over medium heat and whisk constantly until completely cooled. Beat butter and remaining sugar together in medium bowl until light and fluffy. Fold in walnuts and cooled yolk mixture. Turn into mold. Cover and refrigerate 6 hours or overnight. Remove from refrigerator and unmold. Serve immediately with Chocolate Sauce. For Chocolate Sauce: Melt butter in medium saucepan over medium heat. Reduce heat and whisk in cocoa and sugar. Gradually add cream, stirring constantly until mixture is smooth and heated through. Serve warm.