Combine butterscotch chips and 1/3 cup sweetened condensed milk in a 2-cup glass measure.

Combine chocolate chips, remaining sweetened condensed milk, and vanilla in a 1-quart glass measure.

Cook chocolate mixture on high for 1 minute, then stir until melted.

Stir in walnuts.

Spread chocolate mixture evenly in an aluminum foil-lined 8-inch square pan.

Melt butterscotch chips on high for 45 seconds, then stir until melted.

Spread butterscotch mixture evenly over the chocolate layer.

Chill for 3 hours, or until firm.

Turn the fudge onto a cutting board.

Peel off the foil.

Cut into squares.

Store covered in the refrigerator.