01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - Melt the butter and chocolate in a heatproof bowl set over a pan of simmering water, stirring until smooth. Remove from heat.
03 - Whisk in granulated and brown sugars until fully combined. Beat in eggs one at a time, then stir in vanilla.
04 - Fold in flour and salt just until incorporated. Pour batter into prepared pan and smooth the top.
05 - Melt butter in a saucepan over medium heat. Add brown sugar and stir until dissolved. Pour in cream and bring to a simmer; cook 2–3 minutes, stirring constantly.
06 - Remove from heat. Stir in pecans, vanilla, and a pinch of salt.
07 - Drop spoonfuls of praline topping over brownie batter. Gently swirl with a knife for a marbled effect.
08 - Bake for 30–35 minutes, until a toothpick inserted in the center comes out with moist crumbs.
09 - Cool completely in the pan on a wire rack. Lift out using parchment overhang, slice into squares, and serve.