Preheat your oven to 350°F (175°C) and line your 9×9 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a large bowl, whisk together the melted butter and sugar until the mixture is smooth and slightly glossy.
Add the eggs one at a time, whisking well after each addition, then stir in the vanilla extract.
Sift the flour, cocoa powder, and salt into the wet ingredients, folding with a spatula just until the streaks of flour disappear; do not overmix.
In a small bowl, combine the caramel bits and heavy cream, microwaving in 20-second intervals until the caramel is smooth and pourable.
Spread half of the brownie batter into the prepared pan and drizzle half of the melted caramel over the top, using a knife to swirl it gently.
Sprinkle half of the chopped walnuts and pecans over the caramel layer.
Spread the remaining batter over the top, then drizzle with the remaining caramel and sprinkle the rest of the nuts.
Bake for 28 to 30 minutes; the edges should be set, but a toothpick inserted into the center should come out with a few moist crumbs attached.
Allow the brownies to cool completely in the pan for at least two hours before lifting them out to slice, as the caramel needs time to firm up slightly.