Beat first 4 ingredients at medium speed with an electric mixer 2 minutes. Pour batter into 3 greased and floured 8-inch round cakepans. Bake at 350° for 15 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, and cool on wire racks. Wrap cake layers in nonstick aluminum foil and freeze 30 minutes. (This step aids in frosting the finished product.)
Unwrap layers. Spread Raisin-Nut Filling between layers, reserving about 1 1/3 cups for top of cake.
Beat frosting mix and 1/2 cup boiling water at low speed 30 seconds. Scrape down sides of bowl; beat at high speed 5 to 7 minutes or until stiff peaks form. Spread frosting on sides of cake. Spread reserved filling on top of cake. Let stand at least 1 hour before serving. Garnish, if desired.