1Heat oven to 350°F. Grease 18x12-inch half-sheet pan with shortening or cooking spray or line with cooking parchment paper.
2In medium bowl, stir cake mix, butter, vanilla and eggs until well blended (batter will be thick). Spread in pan (you may need to use greased hands to spread evenly). Bake 12 minutes or until set but still soft. Cool completely in pan on cooling rack.
3Meanwhile, line 12x9-inch pan with cooking parchment or waxed paper. In large bowl, stir ice cream, blueberries and strawberries. Spread in pan. Freeze until firm, about 2 hours.
4Cut cake into 6 rows by 4 rows for 24 (3-inch) squares. Remove ice cream from freezer. Cut into 4 rows by 3 rows for 12 (3-inch) squares. Sandwich 1 ice cream square between 2 cake squares. Serve immediately, or wrap individually in plastic wrap or foil and store in freezer.