Preheat oven to 350F.
Lightly spray an 8-inch square baking pan with cooking spray. Line with parchment paper leaving an overhang for easy removal. Set aside.
Cut butter into small cubes.
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.)
Stir in sugar and vanilla until completely combined.
Add eggs one a time, stirring in between until combined.
In a small bowl, whisk together cocoa powder, cornstarch, and 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.
Pour brownie batter into the prepared pan and smooth the top with a spatula.
Bake for 33-37 minutes or until an inserted toothpick comes out with a few moist crumbs.
Let brownies cool completely on a cooling rack.
Use the parchment paper overhang to pull the brownies out and cut into squares.
Store in an airtight container for up to 3 days.