Line 8-inch square pan with enough foil to overhang edges by 2 inches; coat with cooking spray.Pour 1 can milk into large microwave-safe bowl; add bittersweet chocolate chips.Microwave on High in 30-second intervals, stirring until smooth.Spread in pan.
Pour remaining 1 can milk into clean microwave-safe bowl; add white chips.Microwave on High in 30-seconds intervals, stirring, until smooth; stir in extracts, nutmeg and salt.Gently spread over mixture in pan.
Refrigerate until set, 6 hours or overnight.Using foil, lift fudge from pan.Cut into squares.If desired, garnish with chocolate shavings.
To freeze:Tightly wrap uncut chilled fudge in plastic wrap and foil; freeze up to 6 weeks.Thaw in wrappings overnight in refrigerator.Cut and garnish as directed.