Stir chocolate chips, sugar, milk and mint extract in top of a double boiler over simmering water until chocolate is melted. Remove from heat.
Whisk egg yolks in a small bowl until blended; whisk into chocolate. Beat egg whites in medium-size bowl with electric mixer on low speed until foamy. Beat on high speed until soft, rounded peaks form. Spoon over chocolate without mixing.
Beat cream in same mixing bowl on low speed until thickened. Beat on medium speed until soft peaks form. Fold chocolate and egg-white mixture into whipped cream until no streaks of white remain.
Cut angel-food cake into 1-inch pieces. Place layer of cake pieces on bottom of an 8- x 3-inch springform pan. (Or you may use an 8-1/ x 2-inch springform pan.) You'll use half the cake. Top with half the mousse. repeat a second layer of cake and remaining mousse in the same manner.
Smooth top evenly with a spatula. Cover with plastic wrap and refrigerate overnight. The dessert may be refrigerated up to 2 days, or it may be frozen, covered with plastic wrap and foil, if desired. Defrost wrapped cake in refrigerator overnight.
To serve: several hours before serving, remove sides of springform pan. Beat remaining whipping cream until stiff. Spoon into pastry bag fitted with 1-inch rosette or star tip and pipe rows of whipped cream going up the sides and 1 rosette in the center. Or, spread whipped cream over top and sides, if desired. Chill at least 4 hours.