1 1/2 tablespoons butter (no substitutes), divided
2 cups semisweet chocolate chips
1 (11.5 ounce) package milk chocolate chips
1 (14 ounce) can sweetened condensed milk
1 teaspoon peppermint extract
1 teaspoon vanilla extract

Line an 11-in. x 7-in. x 2-in. pan with foil and grease the foil with 1-1/2 teaspoons bitter; set aside. In a heavy saucepan, heat the chocolate chips and remaining butter over low heat until melted; stir until smooth. Remove from the heat; stir in the milk and extracts until well blended. Spread into prepared pan. Refrigerate until set.
Using the foil, lift fudge out of the pan. Discard the foil; cut fudge into 1-in. squares. Store in the refrigerator.