Preheat oven to 350°F and line a metal 9x13-inch metal cake pan with parchment paper and spray lightly with cooking spray. Make sure some of the parchment is over the edge of the pan for easy removal. This may require two sheets, one going in each direction of the pan - make sure the one that goes across the shorter distance is under the one that goes along the full length of the pan.
Melt 1 cup salted butter in a heavy-bottomed saucepan over low heat. Add in 2¼ cups granulated sugar and stir until smooth and the butter and sugar have started to emulsify and no butter is separated on top of the sugar. Remove from heat, transfer to a large bowl, and let the mixture cool for 5 minutes, stirring often to release heat.
Add 2 tablespoons canola oil and 1 tablespoon vanilla extract to the butter and sugar and beat them in with a rubber spatula until fully combined and there's no separation between the ingredients (no oil resting on top).
Mix in 5 large eggs one at a time, beating with the spatula or a fork after each one. Mix until fully incorporated.
Sift the 1 cup all-purpose flour, ¾ cup + 1 tablespoon Dutch-process cocoa powder, and ¾ teaspoon salt into the butter and sugar mixture, stirring gently and minimally, until almost all the dry streaks are gone.
Fold in the ½ cup semi-sweet mini chocolate chips.
Pour the brownie batter into the prepared baking pan and bake for 20 minutes.
While the brownie layer bakes, combine 1½ cups dark brown sugar, ¼ cup salted butter, and ¼ cup dark corn syrup in a medium saucepan and cook over medium heat until the butter has completely melted, stirring frequently.
In a small prep bowl, stir together the 1 tablespoon water and 2 teaspoons cornstarch. Add it to the butter mixture along with 1 tablespoon vanilla extract and ½ teaspoon salt. Bring the mixture to a rolling boil and remove from heat. The center should be vigorously boiling when you remove it from the heat.
Add the 3 large eggs to a large bowl or stand mixer fitted with a whisk attachment and beat just until frothy, about 1 minute.
Slowly add the butter mixture while beating the egg mixture on medium speed to incorporate the two together and temper the eggs.
Add in 1½ cups pecan pieces and 1 cup pecan halves and mix to combine.
Then, pour the mixture over the top or the partially cooked brownies.
Bake for an additional 22 to 25 minutes until the center is slightly jiggly.
Let the brownies cool for 6 hours so the top layer sets completely before removing the brownies from the pan and slicing them with a large sharp knife into squares. If you do not let the brownies set fully, the texture can be a bit too gooey.