Preheat an oven to 350 degrees F (175 degrees C). Coat a 9x13 inch baking pan with cooking spray.
Mix the cake mix, melted butter, and half of the evaporated milk in a large bowl. Pat half of the mixture into the bottom of the prepared pan, reserve the remaining half for later use. Bake for 6 to 8 minutes. Remove baking pan from the oven; immediately sprinkle with toffee chips.
Place the caramels and the remaining half of the evaporated milk in a microwave safe bowl. Microwave on High in 1 minute intervals, stirring after each minute, until caramels are melted. Top the toffee chips with the melted caramel. Crumble the reserved cake mixture over caramel layer.
Return the baking pan to the oven; bake for 15 minutes. Allow to cool completely before cutting into 24 bars.