Line a 9 x 13-inch rimmed sheet pan with parchment paper, leaving a slight overhang on two sides and set close by your stovetop.
In a medium, heavy bottomed saucepan, combine butter, sugar and water. Set over medium heat and bring to a boil. As the mixture comes up to a boil, swirl the pan occasionally to ensure the ingredients are cooking evenly and are well combined.
Once the mixture reaches a boil, place a thermometer into the pan and heat the mixture over medium low heat until it reaches 300˚F. This is known as the hard crack stage and will take 20-25 minutes. As soon as it reaches 300˚F, remove from heat, swirl the pan carefully, then pour onto the prepared sheet pan. Use an offset spatula to spread the toffee into an even layer and get rid of any air bubbles.
Scatter the chocolate chips on top and cover with tin foil, or another sheet pan. Let sit for 5 minutes, or until melted, then use the offset spatula to smooth the chocolate into an even layer.
Scatter the dates and peanuts on top, as well as a sprinkle of flaky salt, if using. Set the pan in the fridge for at least 30 minutes, or until the chocolate is set. Remove the toffee from the pan using the parchment paper as a sling and place on a cutting board. Using a very sharp knife, cut the toffee into 24 equal-sized pieces.