Place sugar and mint in a small food processor; cover and process until blended. Set aside.
Preheat oven to 325 degrees F.
In a large bowl, cream butter and 1-3/4 cups confectioners sugar until light and fluffy. Beat in 4-1/2 teaspoons reserved mint mixture. Add eggs, one at a time, beating well after each addition. Add mashed potato flakes alternately with 1/4 cup cream. Stir in lemon juice and peel.
Pour into a greased and floured 9 x 13-inch baking dish. Bake for 60-70 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
In a large bowl, beat remaining cream until it begins to thicken. Add 1/2 cup confectioners sugar and 1 tablespoon mint mixture; beat until stiff peaks form. Fold in lemon curd; set aside.
In another bowl, combine strawberries and blueberries with remaining mint mixture and confectioners sugar.
Cube cake into 2 squares. Layer in glassware starting with a dollop of the cream mixture, a spoonful of berries and a couple cake cubes. Repeat.