Line an 8-inch by 8-inch square pan with foil, extending foil over edges of the pan.
Melt chocolate chips with sweetened condensed milk and salt in heavy saucepan over medium-low heat to avoid scorching the milk and over-heating the chocolate. Remove from heat. Stir in crushed peppermints and peppermint extract. Spread evenly into prepared pan.
Chill for 2 hours or until firm. Remove from pan by lifting edges of foil. Cut into 1-inch squares.