Before getting started, line a large baking sheet with parchment paper or a silicone baking sheet.
Combine butter, sugars, water, and salt in a large, heavy saucepan. Heat on low, stirring until butter is melted, and all ingredients are well combined.
Raise heat to medium, bring to a boil, and affix a candy thermometer to the side of the pan. Cook until temperature reaches 285 degrees F (137 degrees C), stirring occasionally.
Once toffee reaches 285 degrees, remove pan from heat and stir in blood orange extract and cranberries.
Working quickly, pour toffee out onto prepared pan. Depending on the viscosity and your own preferences, you may want to spread the toffee out a bit, to an even thickness.
If using chocolate chips, allow toffee to cool for a minute or two. Carefully remove any excess butter from the top of the toffee, using a paper towel. Sprinkle chocolate chips over surface of toffee. Allow chips to melt, then use a spoon to spread the chocolate smoothly and evenly over the toffee.
Chill toffee for 15 minutes or so, until set. Break into pieces, and store in an airtight container*.
*This is important – if the toffee is not stored properly, it will absorb moisture from the air and become gooey.