Preheat oven to 350°F. Line a 9-inch square pan with a parchment paper sling.
In a large microwave-safe bowl, combine the bittersweet chocolate and butter. Microwave on high in 20-30 second bursts, stirring well between bursts of heat until melted and smooth. Whisk in cocoa powder.
Add granulated sugar and brown sugar, whisking well to combine. Add eggs and vanilla, beating well either by hand with a whisk or using an electric mixer.
Sprinkle the flour and salt over the brownie batter and gently stir it in with a wooden spoon or spatula. Stir in chocolate chips.
Pour the brownie batter into the prepared baking dish and spread into the corners. The batter will be thick.
Bake the brownie base for 20 minutes until partially set.
Meanwhile, combine brown sugar, eggs, corn syrup, melted butter, vanilla, and salt in a large bowl. Whisk well, then stir in chopped pecans.
Once the brownies are partially baked, remove them from the oven and pour the pecan pie filling over the partially baked brownie layer.
Return the brownie pan to the oven and continue to bake for another 30 to 35 minutes, until mostly set with just a slight jiggle in the center when you gently shake the pan.
Cool completely before cutting. Store in an airtight container for up to 5 days on the counter or in the fridge.