1 cup (2 sticks) unsalted butter, plus more for greasing baking dish
12 ounces bittersweet chocolate, finely chopped
2 cups all-purpose flour
1/4 teaspoon cayenne pepper, or less to taste
1/2 teaspoon coarse salt
1 3/4 cups sugar
4 large eggs
1 tablespoon pure vanilla extract
6 ounces semisweet chocolate chunks
3 ounces chopped pecans (about 1 cup)
1 cup sugar
1/4 cup water
1/2 cup heavy cream
1 small vanilla bean, split in half lengthwise
1 teaspoon freshly squeezed lemon juice
1 tablespoon unsalted butter
Preheat oven to 350 degrees. Butter a 9x13-inch glass baking dish. Line bottom of pan with parchment paper; butter the paper. Set aside.
Melt bittersweet chocolate and butter in a double boiler over barely simmering water. Stir occasionally until smooth and creamy. Remove from heat and set aside.
Sift together the flour, cayenne, and salt, and set aside. Place the sugar and eggs in the bowl of an electric mixer fitted with the paddle attachment, and beat until fluffy and well combined, 3 to 5 minutes. Add the vanilla extract and chocolate mixture, and stir to combine. Add the dry ingredients, and stir to combine. Stir in melted chocolate. Pour into dish, and sprinkle with chopped pecans. Bake until set, 35 to 40 minutes. Transfer the pan to a wire rack to cool.
Meanwhile, make the caramel sauce. Stir together the sugar and the water in a 2-quart saucepan over medium heat. Without stirring again, cook until the mixture is dark amber in color, swirling pan carefully and occasionally while cooking, about 20 minutes.
Reduce the heat to low. Slowly and carefully add the cream, stirring with a wooden spoon when the bubbling dies down. Scrape the vanilla seeds into pan; add the pod. Add the lemon juice and butter. Stir to combine. Remove pod. Drizzle over cooled brownies before cutting into bars.