Line the bottom and sides of a 9x9 baking pan with parchment paper or aluminum foil. If using parchment paper, a little cold butter on the pan will help keep the parchment paper in place. Preheat your oven to 350F.
In a double boiler (heatproof bowl set over a sauce pan with barely simmering water) melt the entire bag of chocolate and the butter, stirring frequently to incorporate. Let cool slightly, about 3 or 4 minutes, enough time to get the remaining ingredients ready.
Using a whisk, add granulated and brown sugars to the melted chocolate.
Whisk in eggs, one egg at a time. Add vanilla extract.
Add flour, cocoa powder and salt. Using a spatula, fold the dry ingredients into the chocolate/sugar mixture.
Pour the batter into the 9x9 pan. Drizzle caramel over batter in rows. Drag a knife or toothpick across the rows, swirling the caramel.
Bake for 35-37 minutes until a toothpick inserted in the center comes out clean. Garnish with coarse natural sea salt.