Method
1. Prep
Preheat the oven to 325°F. Line an 8x8-inch metal pan with parchment paper, leaving an overhang.
2. Mix the Wet Ingredients
Melt the butter in a large microwave-safe bowl (about 1 minute). Whisk in the granulated and brown sugars, then let cool to just warm. Whisk in the eggs, egg yolk, and vanilla vigorously for 30 seconds. Whisk in the oil and cocoa powder.
3. Add the Dry Ingredients
Stir in the flour, cornstarch, and salt until just combined. Fold in the chocolate chips and toffee bits, reserving a handful of each.
4. Bake
Spread the batter evenly into the prepared pan. Sprinkle with the reserved toffee bits and chocolate chips. Bake for 30 minutes, or until set with moist crumbs on a cake tester (add 5-7 minutes for a less fudgy brownie). Let cool completely before cutting.
Cook's Notes
Cocoa powder Dutch-process gives the smoothest, richest flavor, but natural cocoa works too.
Storage Airtight up to 3 days at room temperature or 5 days in the fridge; freezes well up to 3 months. Chilled brownies are extra fudgy.