Preheat oven to 350F. Grease an 8- or 9-inch square baking dish with butter. Line with parchment, leaving an overhang on two sides for easy removal. Set aside.
Sprinkle pecans on a dry sheet pan. Roast for 5-7 minutes or until fragrant. Set aside.
Make the brownie batter. In a small bowl, combine cocoa powder, cornstarch and salt and use a fork to combine. Set aside.
Fill a small saucepan with 1 to 2 inches of water. Place a heatproof bowl over the top, making sure the water does not touch the bowl's bottom. Remove the bowl and bring the water to a boil.
Add the butter and chocolate to a heatproof bowl. When the water is boiling, turn the heat to low and place the heatproof bowl over it. Use silicone spatula or wooden spoon to mix frequently until melted and smooth. Add sugar and whisk for 1 minute. Take off the heat. Stir in dry ingredients.
In a small mixing bowl, combine eggs and vanilla. Beat with a whisk or fork until slightly bubbly, about 1 minute. Stir into chocolate mixture until smooth.
Pour the batter into the prepared pan. Tap the full pan on the countertop five times to release air bubbles. Bake for 15 minutes until the top has set.
Fill pecan pie. In a medium saucepan, whisk together maple syrup (or corn syrup), brown sugar, eggs, vinegar, vanilla, nutmeg, and salt. Add butter. Whisk constantly over medium heat until bubbles form around the edges (about 3-5 minutes). The mixture will barely thicken.
Place a strainer over a heatproof bowl. Pour the filling mixture through to remove any bits of cooked egg. Fold in pecans.
Spoon pecan pie filling over partially baked brownies. Bake an additional 30-35 minutes or until center, barely jiggles when the pan is jostled.
Let brownies cool completely in pan on a cooling rack. Refrigerate for 1-2 hours to cut cleanly.
Use a parchment overhang to remove brownies from a cutting board. Cut into 16-25 pieces and serve.
Leftovers will keep an airtight container for up to 4 days at room temperature or up to a week in the refrigerator. These can also be covered with wax paper or parchment and frozen.
