Line a 7" by 11" baking pan with parchment paper. Set aside.
Add butter, granulated sugar, brown sugar and cream to a saucepan. Heat, stirring with a wooden spoon, until the sugar is fully dissolved.**
Stop stirring, stick a candy thermometer into the saucepan and boil the mix, untouched, until it reaches the hard crack stage (300 to 310 degrees F.)*** (Be careful, 300 degrees F. is extremely hot and dangerous. Do not, under any circumstances, touch the caramel OR the pot).
Take off the heat and, very carefully, standing back, stir in the vanilla.
Pour the hot mix into the baking pan and let sit for a minute.
Mix the chopped chocolate, then evenly distribute it over the hot toffee and let it sit until it starts to melt.
Evenly spread the chocolate over the toffee using an offset spatula or a rubber spatula.
Sprinkle with fleur de sel.
Once the chocolate has solidified, cut the toffee into pieces.
*Please be sure to use baking chocolate bars and not chocolate chips or chocolate morsels for this glaze. Chocolate chips and morsels are often designed to keep their shape when heated and therefore have stabilizers in them. Because of that they don't melt as well as well as chopped bar chocolate.**You can tell that the sugar is completely dissolved when you stop hearing it crunch and can't feel any grittiness in the saucepan any more as you're stirring with the wooden spoon.***It will take a while to get to 300 degrees F. The mix will heat up to around 200 degrees F. quickly and then very slowly move to 300 degrees.