Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the long sides to form a sling. Lightly spray the parchment with nonstick spray or brush with melted butter.

Spread the chopped pecans on a baking sheet and toast in the preheated oven for 5–7 minutes, until fragrant and slightly darkened. Set aside to cool while you make the batter.

In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until glossy and well combined. Add the eggs one at a time, whisking well after each addition, then whisk in the vanilla. In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder until no cocoa streaks remain. Gently fold the dry ingredients into the wet mixture just until combined; the batter will be thick and shiny.

Spread about 2/3 of the brownie batter into the prepared 9×13-inch pan, smoothing it into an even layer. Reserve the remaining 1/3 of the batter. Bake for 15–18 minutes, until the edges look set and the center is no longer liquid but still soft.

While the base bakes, place the unwrapped caramels and heavy cream in a medium saucepan over low heat. Stir constantly for 5–7 minutes, until the caramels melt into a smooth sauce. Remove from heat as soon as it is smooth and avoid letting it boil hard so the caramel stays soft. (Alternatively, melt in the microwave in 30-second bursts, stirring between each, until smooth.)

When the brownie base is done, remove the pan from the oven. Sprinkle about 1 cup of the toasted chopped pecans evenly over the warm brownie layer. Slowly pour the warm caramel sauce over the pecans, tilting the pan gently to coat the surface. Dollop the reserved 1/3 brownie batter over the caramel in small spoonfuls, then lightly swirl with a butter knife.

Return the pan to the oven and bake for 18–22 minutes more, until the edges are set and the center looks just slightly jiggly but not wet. A toothpick may pick up some caramel, but it should not come out with raw brownie batter.

As soon as the brownies come out of the oven, place the chocolate chips and 2 tablespoons butter in a microwave-safe bowl. Heat in 20–30 second bursts, stirring between each, until smooth and fully melted. Pour the melted chocolate over the warm brownies and gently spread into an even layer. Sprinkle the remaining toasted pecans (about 1/2 cup) over the top and finish with a light sprinkle of flaky sea salt.

Let the brownies cool at room temperature for about 1 hour. For clean slices and the best texture, chill in the refrigerator for at least 2 hours, or overnight. Use the parchment sling to lift the slab from the pan, then cut into 16 large or up to 24 smaller squares with a sharp knife, wiping the blade between cuts.

Serve at room temperature or slightly chilled. Store leftover brownies in an airtight container at room temperature for up to 2 days, or in the refrigerator for up to 5 days. For longer storage, freeze slices on a baking sheet until solid, then transfer to an airtight container and freeze for up to 2 months.