Heat oven to 350°F (325°F for dark or nonstick pans). Grease bottoms only of two 8- or 9-inch round cake pans with shortening or cooking spray. In large bowl, beat cake mix, buttermilk, oil, lemon peel and eggs with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour into pans.
Bake 8-inch rounds 30 to 36 minutes, 9-inch rounds 28 to 33 minutes, or until toothpick inserted in center comes out clean. Cool 10 minutes. Run knife around side of pans to loosen cakes; remove from pans to cooling racks. Cool completely, about 1 hour.
Place 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.
In 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.
***For a smooth mousse, strain the raspberry seeds out of the pie filling. Place the filling in a mesh strainer or sieve over a bowl and use the back of a spoon to press the filling through the sieve.**.