Preheat oven to 350 degrees F.
Line a 9-inch-square baking pan with parchment paper, leaving an overhang. Butter the paper.
In a mixing bowl, beat 9 T. butter and the brown sugar on medium speed until light and fluffy. Add the flour and 1/4 teaspoons salt, and beat until just combined. Press the dough evenly in the pan and bakc until lightly browned, about 30 minutes.
Place the chopped chocolate in another bowl.
Heat the sugar and water in a saucepan over medium-high head until it turns amber in color, about 10 minutes. Remove from heat and add 6 T. butter, the cream, and the remaining 1/2 teaspoons salt.
Pour the mixture over the chopped chocolate. Let it stand for 2 minutes, then stir to combine and let it stand 10 minutes longer.
Pour the chocolate mixture over the crust and refrigerate at least 4 hours or overnight.
Lift the parchment paper to remove the whole bar from the pan.
Sprinkle with the sea salt, then trim edges and cut into 24 bars. The mixture may be sticky; wipe the knife often while cutting.
Can be stored, refrigerated, for up to three days.