1Heat oven to 350°F (325°F for dark or nonstick pans). Grease or spray two 8- or 9-inch round cake pans. In large bowl, beat cake mix, buttermilk, oil, lemon peel and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour into pans.
2Bake as directed on box. Cool 10 minutes. Run knife around side of pans to loosen cakes; remove from pans to cooling racks. Cool completely, about 1 hour.
3Place 1 cake layer, rounded side down, on serving plate. Spread 3/4 cup of the pie filling over cake layer to within 1/4 inch of edge. Top with second layer, rounded side up.
4In chilled medium bowl, beat whipping cream with electric mixer on high speed until soft peaks form. Beat in remaining 1 1/4 cups pie filling on low speed just until blended. Frost side and top of cake with raspberry mousse. Garnish with fresh raspberries and mint leaves. Store loosely covered in refrigerator.