In a medium saucepan, combine the granulated sugar, water, light corn syrup, and salt. Heat over medium heat, stirring gently until dissolved. Boil for about 5-8 minutes until amber. Whisk in warmed heavy cream with scraped vanilla bean, followed by lemon juice and butter, one piece at a time, whisking until smooth. Let cool completely.

Preheat oven to 350°F (175°C). Butter a 9x13-inch baking dish and line with parchment paper. Melt salted butter with chopped bittersweet chocolate in a microwave-safe bowl until smooth. In another bowl, whisk eggs with granulated and dark brown sugars, salt, and vanilla until thick and pale, about 4-5 minutes.

Gently fold the melted chocolate mixture into the egg mixture until just combined. Sift in flour and cocoa powder, folding carefully. Pour the brownie batter into the prepared dish, smoothing the top. Bake for about 25 minutes, or until slightly cracked and a toothpick comes out with a few moist crumbs. Cool completely.

In a mixing bowl, whip mascarpone and heavy cream until combined. Gradually add sugar, vanilla bean paste, and salt, beating until soft peaks form. Gently fold in cooled caramel, reserving 1/4 cup for drizzling later. Spread topping evenly over cooled brownies.

Drizzle reserved caramel over the mascarpone layer. Chill brownies in the refrigerator for 1-2 hours to set. Cut into squares and enjoy.