When in doubt, use weight over volume measures.
Use an instant-read or candy thermometer when cooking your toffee.
If the butter separates while you cook your toffee, add a drop of hot water and whisk vigorously until the mixture comes back together.
If your chocolate doesn’t melt on the hot toffee, pop your baking sheet under the broiler for a few minutes until the chocolate chips are starting to look shiny.