Step 1

Preheat oven to 350°. Coat an 8x8x2-inch baking pan with nonstick spray. Dust with cocoa powder, tapping out excess. Place butter, unsweetened chocolate, and bittersweet chocolate in a medium microwave-safe bowl. Heat in microwave in 15-second intervals, stirring occasionally, until melted. Let cool slightly.

Step 2

Meanwhile, whisk flour, espresso powder, kosher salt, and 1 tablespoon cocoa powder in another medium bowl; set aside.

Step 3

Using an electric mixer on high speed, beat eggs and sugar in a large bowl until pale and thick, about 2 minutes; beat in vanilla. Fold in warm chocolate mixture, then dry ingredients, just until combined. Fold in chocolate chips. Scrape batter into prepared pan and smooth top.

Step 4

Bake brownies until a tester inserted into the center comes out with some moist crumbs attached, 20-25 minutes. Transfer pan to a wire rack; let brownies cool in pan. Cut brownies into 16 squares.

Step 5

Bring sugar and 2 tablespoons water to a boil in a small saucepan over medium heat, stirring until sugar dissolves. Increase heat to medium-high and cook without stirring, occasionally swirling pan and brushing down sides of pan with a wet pastry brush, until sugar is a deep amber color, 5-8 minutes. Remove caramel from heat.

Step 6

Add cream and butter (mixture will bubble vigorously) and whisk to combine. Let cool slightly in pan.

Step 7

Drizzle caramel sauce over brownies and sprinkle with sea salt. DO AHEAD: Brownies can be made 1 day ahead. Cover and keep at room temp-erature. Caramel sauce can be made 1 week ahead. Let cool. Cover and chill. Reheat before using.