1. Preheat oven to 350°F and line a 9×13-inch pan with parchment paper leaving overhang on all sides.

2. Melt 1 cup butter and 8 ounces chocolate together until smooth, then cool for 5 minutes.

3. Whisk in 1½ cups granulated sugar and ½ cup brown sugar until combined.

4. Add eggs one at a time, beating well after each addition, then stir in 2 teaspoons vanilla.

5. In separate bowl, whisk together flour, cocoa powder, salt, and baking powder.

6. Fold dry ingredients into chocolate mixture just until combined and pour into prepared pan.

7. Bake for 28-32 minutes until toothpick inserted near edge has moist crumbs. Cool completely for 2 hours.

8. For caramel, combine 2 cups sugar and water in heavy saucepan over medium heat, stirring until sugar dissolves.

9. Stop stirring and cook until deep amber, about 12-15 minutes, swirling pan occasionally.

10. Remove from heat and carefully whisk in warm cream, then add 6 tablespoons butter, 1 teaspoon vanilla, and sea salt.

11. Let caramel cool for 15 minutes, then pour over cooled brownie base and refrigerate for 1 hour.

12. For ganache, place 12 ounces chopped chocolate in heatproof bowl.

13. Heat 1 cup cream until just simmering and pour over chocolate. Let sit 3 minutes undisturbed.

14. Whisk ganache from center outward until smooth, then add 2 tablespoons butter and 1 teaspoon vanilla.

15. Let ganache cool for 10 minutes, then pour over set caramel layer and spread evenly.

16. Refrigerate for 2 hours until ganache firms up completely.

17. Sprinkle with flaky sea salt, then cut into squares using a hot, dry knife wiped clean between cuts.