Prepare and bake cake according to package directions, using a 9-in. round baking pan. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
In a small bowl, sprinkle gelatin over 1/4 cup milk; let stand for 1 minute. In a small saucepan, combine the sugar, coffee granules, salt and remaining milk. Cook and stir until bubbles form around edge of pan. Add gelatin mixture; stir until dissolved. Transfer to a bowl. Refrigerate until slightly thickened, about 1-1/4 hours.
In a chilled bowl, beat cream until soft peaks form. Fold whipped cream and 1/2 cup walnuts into gelatin mixture.
Line the bottom of a greased 9-in. round pan with waxed paper. Pour gelatin mixture into pan; carefully place cooled cake layer on top. Refrigerate for at least 3 hours.
Just before serving, invert dessert onto a serving platter; gently peel off waxed paper. Garnish with remaining walnuts.