Preheat oven to 350°F and grease a metal 12×17-inch sheet pan with nonstick cooking spray.
Melt 1 cup unsalted butter in a heavy-bottomed saucepan over low heat.
Add 2½ cups granulated sugar and stir until smooth, the butter and sugar have started emulsifying, and no butter is separated from the sugar. Remove from heat and let the mixture cool for 5 minutes, stirring often to release heat.
Add 2 tablespoons canola oil and 1 tablespoons 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).
Beat in 5 large eggs one at a time, beating with the spatula or a fork after each one. Mix until fully incorporated.
Sift 1 cup all-purpose flour, ¾ cups + 1 tablespoon unsweetened cocoa powder, 2 tablespoons powdered milk, and ¾ teaspoon salt into the butter and sugar mixture, stirring gently with a rubber spatula just until the dry streaks are gone.
Fold about two thirds of the 8 ounces dark chocolate into the batter.
Pour the brownie batter into the prepared baking pan, sprinkle the remaining chunks on top, and bake for 16 to 19 minutes.
Cool before removing it from the pan and slicing it for best results. Use a plastic knife for the cleanest cuts.