Line an 8-inch square baking pan with aluminum foil.

In a small saucepan combine the butterscotch chips with 1/3 cup of sweetened condensed milk. Set aside for the moment.

In a medium saucepan over medium heat combine the remaining sweetened condensed milk and semi-sweet chocolate chips. Once the mixture begins to melt, stir continuously until all the chips are melted, the ingredients are incorporated and smooth. Remove from heat and add vanilla stirring quickly to incorporate.

Spread the chocolate mixture into the prepared pan. Set aside.

Repeat the same method for melting above for the butterscotch chips and sweetened condensed milk, stirring until mixture is incorporated and melted. Spread evenly over chocolate layer.

Refrigerate until firm at least 2 - 3 hours.