Preheat the oven to 350°.
Heavily line a half sheet pan with foil and grease corner to corner with a little butter.
Lay the crackers out onto the sheet pan, in an even layer.
Heat the butter and brown sugar in a saucepan, over medium heat, stirring occasionally, until the mixture comes to a boil.
Pour the toffee over the crackers, and then use a spatula to smooth it out over the entire surface.
Sprinkle with the milk chocolate chips and transfer to the oven to bake for about 5 minutes. Remove the pan from the oven and let it cool for a minute.
Now the chocolate chips have begun to soften and melt. Use an off-set spatula to spread the melted chocolate evenly over the toffee.
Sprinkle with about a teaspoon or more to taste of the large flaked sea salt, if you choose.
Once the pan has cooled slightly, place it in the fridge until the toffee and chocolate are set... about 30 minutes. Break it up into any size pieces you'd like and serve. The toffee can be stored in an airtight container (either refrigerated or in a cool dry spot) for up to one week.