Line 9" square pan with foil; coat with cooking spray. In microwave-safe bowl, combine 3 cups chips, 1 can milk and 1/4 tsp. salt. Microwave on High in 30-second intervals, stirring until smooth. Stir in vanilla; spread in pan..
In separate microwave-safe bowl, combine remaining chips, milk and salt. Microwave on High in 30-second intervals, stirring, until smooth. Stir in strawberry extract and food coloring. Spread over vanilla layer.
Slightly swirl layers together. Cover; chill until set, 4 hours. Remove from pan; cut into 1" squares.