Prepare, bake and cool cake as directed on package. Cut cake into 1-inch cubes. Drain thawed strawberries, reserving juice.
Combine gelatin and boiling water in small bowl. Stir until gelatin is dissolved. Add enough water to strawberry juice to measure 1 cup; stir into gelatin. Refrigerate until gelatin is slightly thickened. Beat gelatin until foamy.
Beat 1 cup whipping cream until stiff peaks form in large bowl. Fold into gelatin along with strawberries.
Alternate layers of cake cubes and strawberry mixture into 10-inch tube pan. Press lightly. Cover. Refrigerate overnight.
Unmold cake onto serving plate. Beat remaining 1-1/2 cups whipping cream, sugar and vanilla extract until stiff peaks form. Frost sides and top of cake. Refrigerate until ready to serve. Garnish with fresh strawberries and mint leaves.