Preheat your oven to 400F (205C) and line a standard-sized cookie sheet with foil. Lightly spray with cooking spray or grease lightly with butter.
Layer crackers evenly over the foil; it's alright if there are spaces between the crackers and not every edge is completely filled in.
Prepare your toffee by combining butter and brown sugar in a small saucepan over medium heat, stirring occasionally.
Allow mixture to come to a boil and allow it to boil for three minutes -- do not stir while it boils.
Immediately remove from heat and pour over crackers (it's ok if some of them start to float, just try to keep them in an even layer), spread with a heatproof spatula to completely coat crackers.
Place in 400F (205C) oven for 5 minutes -- toffee will begin to bubble.
Remove from oven and evenly sprinkle chocolate chips over toffee. Return to oven for two minutes, then remove and use a spatula or knife to spread chocolate evenly over crackers.
Sprinkle evenly with pecans and a scant amount of flaky sea salt.
Allow to chill in refrigerator for at least 30 minutes or until chocolate hardens. Cut into pieces using a large knife and serve.
Cleanup tip: Did the toffee harden to your saucepan, rendering it virtually impossible to clean? Fill it about halfway with water and return it to the stovetop. Bring the water to a boil and the toffee should melt away from the sides -- then just pour everything down the drain (scrape the sides while it's hot, if there's still some toffee stuck) and allow the pot to cool and clean as you would normally.