Preheat the oven to 180°C. Grease an 8-inch square baking pan with enough parchment paper to overhang the edges.
In a heatproof bowl set over a saucepan of barely simmering water, melt the baking chocolate, butter and salt, stirring occasionally.
Remove from the heat and stir in the sugar. Allow to cool slightly.
Stir in the egg until well combined, then stir in vanilla. Fold in the flour until combined and stir in half of the chopped TWIX® Twin Bars.
Scrape the brownie batter into a prepared pan and smooth the top. Bake for 20 to 25 minutes or until only a few moist crumbs adhere to a toothpick inserted in center.
Allow to cool completely, remove from pan and remove the parchment paper.
Stir together the buttercream frosting and 2 tbsp dulce de leche until blended, and spread over the brownies.
Sprinkle brownies with the remaining chopped TWIX® Twin Bars drizzle the remaining dulce de leche on top. Cut into 16 squares.
Tip: Substitute caramel sauce for dulce de leche.