Line a 15x10 - inch baking pan with foil and lightly spray with cooking spray. (This is a jellyroll size pan.) Set aside.

In a heavy bottomed saucepan, combine the butter, sugar, and salt. Melt the butter over medium-high heat, stirring until the sugar is completely dissolved.

Clip a candy thermometer to the saucepan and continue cooking and stirring the mixture until it reaches 290 degrees Fahrenheit.

Stir in the vanilla extract and slivered almonds; pour toffee into the prepared pan. Top the pan to spread the toffee evenly. Place the pan on a rack to cool.

In a small bowl combine butterscotch chips and half of the shortening. Combine the milk chocolate chips and the remaining shortening in a separate bowl.

Heat on high in the microwave in 30 second intervals, stirring in between, until the chips have fully melted and are smooth.

Use an offset spatula to spread the butterscotch and chocolate on top of the cooled toffee.

Refrigerate for at least 30 minutes to set the chocolate.

Break into pieces and serve.