Line a 9x13 pan with parchment paper and set aside. In a medium-sized heavy-bottomed sauce pan with a candy thermometer attached, melt butter, both sugars, molasses, salt, and espresso together over medium-high heat.
Cook, stirring occasionally with a whisk, until the temperature approaches 250 degrees; then stir constantly until it reaches 300 degrees.
Pour immediately into the prepared pan- spread evenly with a spatula. Sprinkle the chocolate chips over the toffee, let them sit for a minute until the heat from the toffee makes them soft. Spread the chocolate evenly over the toffee base. Sprinkle the topping (pretzels, nuts, etc.) onto the melted chocolate. Put the toffee into the fridge to harden. Once hard and set, break it into pieces with you hands. Store in an air-tight container in the fridge for up to a week.
