Start by making the toffee. Put butter, light brown sugar, and kosher salt in a sauce pan over medium-low heat. You can cut the butter into smaller pieces so it melts easier. Mix together, continuously stirring. Heat until it reaches 300 degrees Fahrenheit (use a candy thermometer to be sure). Pour the toffee onto a baking sheet with parchment paper, then put it in the freezer to harden. In a medium-size bowl, mix the brown and white sugar, salt, espresso, and baking soda for the dough. Next, make brown butter by heating two sticks of butter on the stove over medium heat. Stir continuously until it turns golden brown. Put two ice cubes in the butter and mix until melted. Pour butter into the dough mixture. Mix in the eggs and vanilla. Add flour to the mixture to thicken the dough. Combine until smooth. Cut the chocolate into chunks with a knife. Set aside. Break the toffee into smaller bits, using a spoon or rolling pin. Pour the chocolate and toffee into the batter, folding over until evenly distributed. Use an ice cream scooper to scoop dough onto a pan with parchment paper. Put in the fridge for 48 hours to let the flavors settle and intensify (it makes a big difference!). After 48 hours, bake the cookies at 300 degrees for about 20 minutes. They should be golden brown with slightly crispy edges. Note: I had to periodically use a spatula to gently shape the cookies so they stayed round. Sprinkle sea salt over the top for the final touch. Serve with milk and enjoy!