1. Heat the oven to 400 degrees Fahrenheit. Line a baking sheet with a silicone baking mat.
2. Spread the miso out in an even layer, about 1/4-inch-thick, on the lined baking sheet. Bake it until the miso is well browned and quite a bit burnt around the edges, about 20-25 minutes. Remove it from the oven and let it cool slightly. Then crumble into small pieces and set aside until ready to use.
3. In a large, heavy-bottomed saucepan, combine the butter and sugars. Cook over medium heat, stirring until the butter is melted.
4. Allow to come to a boil. Cook until the mixture becomes a dark amber color and the temperature has reached 285 degrees Fahrenheit (137 degrees Celsius). Stir constantly. Meanwhile, cover a large baking sheet with aluminum foil or parchment paper.
5. When the toffee reaches the correct temperature, add the mirin, salt, and vanilla. Stir to integrate.
6. Add the miso and stir it until the miso is evenly distributed.
7. Pour the butterscotch over the pan and allow it to cool for up to five minutes. Sprinkle the chocolate chips on top, and when they begin to melt, spread them over the toffee layer. Then sprinkle nuts or miso toasted nuts (instructions below).
8. Place the toffee in the refrigerator to chill until set. Break into pieces, and store in an airtight container.