Brownies with Salted Caramel Sauce

INGREDIENTS

• Nonstick vegetable oil spray
• 1 natural unsweetened cocoa powder plus more for pan
• 1/2 (1 stick) unsalted butter, cut into 1/2-inch pieces
• 3 unsweetened chocolate, chopped
• 2 bittersweet chocolate, chopped
• 2/3 all-purpose flour
• 1 instant espresso powder
• 1/4 kosher salt
• 2 large eggs
• 1 sugar
• 1 vanilla extract
• 1/3 bittersweet chocolate chips
• 1/2 sugar
• 2 heavy cream
• 2 unsalted butter, cut into 1/2-inch pieces
• Gray sea salt or flaky sea salt (such as Maldon; for serving)