From my sister and a family cookbook.
1 can sweetened condensed milk
1 TB butter
16oz milk chocolate (Symphony bars are great)
2 oz semi-sweet chocolate (do not use cheap chocolate chips, they have too much was in them)
½ tsp peppermint extract, or more
In a double boiler milk the first four ingredients together until smooth. Remove from heat and add the peppermint extract.
Spray an 8X8 pan. Pour chocolate into the pan. Place in fridge until set, 2 to 3 hours. Cut into squares.