Preheat oven to 350°F.
Line an 8-inch x 8-inch baking pan with parchment paper, leaving a 2-inch overhang on two sides. Butter parchment paper and exposed edges of pan, set aside.
In a medium bowl, whisk together flour, cocoa powder and salt.
In a large bowl, combine melted butter, sugar and brown sugar. Whisk until well combined. Add eggs one at a time, whisking vigorously until combined. Whisk in vanilla. Mixture should be glossy and smooth. Add dry ingredients and stir until just combined.
Pour two-thirds of batter into prepared pan. Bake for 12 minutes. Gently drizzle caramel sauce on surface, then dollop remaining batter over top. Use a skewer or sharp knife to gently marble batter. Tap pan against counter to even out surface, then bake another 40-45 minutes, or until edges begin to pull away from the pan and a toothpick inserted in the center comes out mostly clean. Let cool completely. Run a sharp knife around the edges to loosen any hardened caramel, then use the parchment paper sling to carefully remove brownies from the pan. Decorate with marshmallow ghosts.
Place gelatin in a small bowl. Cover with a few tablespoons of the water and set aside to soften.
Place egg white in the bowl of a stand mixer fitted with a whisk attachment. Whisk by hand until frothy and expanded enough for mixer to reach it, then use the machine to beat it to soft peaks.
Heat remaining water, sugar and corn syrup in a medium pot over medium heat. Cook, swirling pot, until sugar has dissolved, then increase heat to medium-high. When syrup reaches 230°F on a candy thermometer, turn on mixer and beat egg whites to stiff peaks; turn off when ready.
Continue to cook syrup until it reaches 262°F. Stir in softened gelatin and vanilla and stir until gelatin has melted.
Turn on the mixer to medium and slowly pour hot syrup over egg whites. Increase speed to high and beat until shiny and the bowl is cool to the touch, 7-10 minutes.
Immediately transfer to a piping bag fitted with a large plain tip (½-inch diameter, or #12 tip). Pipe ghosts on the surface of the cooled brownies (work quickly, marshmallows will begin to set as they cool). When set, use the store-bought black decorating gel to make eyes and mouths on the ghosts.