Preheat oven to 350°F (177°C). Line a 9x13-inch baking pan with parchment paper, leaving extra paper on the sides for easy lifting.

In a large saucepan over medium heat, melt 1 cup butter. Remove from heat and whisk in 2 cups granulated sugar until smooth.

Add 4 eggs (one at a time), whisking continuously until fully incorporated. Stir in 1 teaspoon vanilla.

Sift in 1 cup flour, 1 cup cocoa powder, and 1 teaspoon salt. Stir until just combined and no streaks remain.

Pour the brownie batter into the prepared pan and spread it evenly with a spatula. Bake for 20 minutes.

While the brownies bake, prepare the pecan pie topping: In a large bowl, whisk together 1 cup brown sugar, 1 cup corn syrup, 4 eggs, 4 tablespoons melted butter, 1 teaspoon vanilla extract, and 1 tablespoon flour until smooth. Stir in 2 cups chopped pecans and 8 ounces chopped semisweet chocolate.

After the brownies have baked for 20 minutes, remove from oven and gently pour the pecan pie mixture evenly over the brownie layer, spreading to cover.

Return the pan to the oven and bake for an additional 20-25 minutes, or until the pecan pie topping is set and the edges are golden brown.

Allow brownies to cool completely in the pan before lifting out and slicing into squares.