In a medium saucepan over medium heat, combine the sweetened condensed milk, 2 tbsp salted butter, 1/2 cup brown sugar and 1/4 cup corn syrup. Heat while stirring constantly until the mixture reaches 225°F (softball stage).

Remove the caramel mixture from heat and stir in the 3 large marshmallows until smooth. Set aside.

Preheat your oven to 350°F. Line an 8x8 inch pan with parchment paper, leaving an overhang for easy removal.

In a large bowl, whisk together the 3/4 cup melted salted butter, 1 cup brown sugar, 1/4 cup granulated sugar, 3 large eggs and 1/2 tbsp vanilla extract until combined.

Fold in the flour, cocoa powder, cornstarch and salt until just combined. Do not overmix.

Spread 2/3 of the brownie batter onto the bottom of the prepared pan.

Pour and spread the prepared caramel evenly over the brownie batter.

Dollop the remaining brownie batter on top of the caramel. Use a toothpick or skewer to gently swirl the brownie batter into the caramel layer.