Preheat oven to 325°F. Line a 9”x13" glass baking dish with foil, pressing firmly into pan and leaving a 2" overhang. Coat foil with nonstick spray; set baking dish aside.

Melt butter in a small saucepan over medium heat or in a microwave. Let cool slightly.

Whisk sugar, cocoa, and salt in a medium bowl to combine.

Pour butter in a steady stream into dry ingredients, whisking constantly to blend.

Whisk in vanilla.

Add eggs one at a time, beating vigorously to blend after each addition.

Add flour and stir until just combined (do not overmix).

Scrape batter into prepared pan; smooth top.

Bake until top begins to crack and a toothpick inserted into the center comes out with a few moist crumbs attached, 30-40 minutes.

While brownies are baking: In a medium heavy-bottomed saucepan, combine heavy cream, butter, brown sugar, and water. Stir over medium heat until the mixture is smooth. Bring to a boil and cook, without stirring, until a candy thermometer registers 240°F, about 5 to 7 minutes.

Carefully pour caramel over hot brownies (just out of the oven).

Sprinkle with coarse or flaky sea salt.

Transfer pan to a wire rack; let cool completely in pan. (They will not cut well if still warm.)

Using foil overhang, lift brownies out of pan; transfer to a cutting board and cut into 32 squares.