Preheat oven to 350°F. Lightly spray an 8-inch square baking pan with cooking spray. Line with parchment paper leaving an overhang for easy removal. Set aside. In a large bowl, combine chocolate chips and butter. Melt in 30-second increments, on high heat in the microwave, stirring in between. (It took 1 minute total for my chocolate to melt.)
12 ounces semi-sweet chocolate chips, 6 tablespoons unsalted butter
Stir in granulated sugar and vanilla extract until completely combined.
¾ cup granulated sugar, 1 teaspoon vanilla extract
Add eggs one a time, stirring in between until combined.
3 large eggs
In a small bowl, whisk together unsweetened cocoa powder, cornstarch and salt.
¼ cup unsweetened cocoa powder, ⅓ cup cornstarch, ½ teaspoon fine sea salt
Add dry mixture to wet mixture and stir vigorously until the mixture is smooth and begins to pull away from the sides of the bowl. This will take one or two minutes.
Stir in the toffee chips if using.
1 cup Heath toffee bits
Pour brownie batter into the prepared pan and smooth the top with a spatula.
Bake for 33 to 37 minutes or until an inserted toothpick comes out with a few moist crumbs. Let brownies cool completely in pan on a cooling rack.
Use the parchment paper overhang to pull the brownies out and cut into squares.