Preheat the oven to 325°F (approximately 163°C). Line an 8 × 8-inch metal baking pan with parchment paper, leaving an overhang on two opposite sides so you can lift out the cooled brownies.

Place the butter in a large microwave-safe bowl. Microwave in short bursts, stirring between each burst, until melted. Whisk in the granulated sugar and brown sugar, breaking up any lumps. Let the mixture cool until just warm.

Add the two whole eggs, the extra egg yolk, and the vanilla extract. Whisk vigorously for 30 seconds until combined.

Whisk in the vegetable oil, followed by the cocoa powder. Mix until no dry cocoa pockets remain, scraping the sides of the bowl as needed.

Add the flour, cornstarch, and salt. Using a rubber spatula, gently fold the dry ingredients into the chocolate mixture. Stop once no dry flour remains.

Reserve a small handful each of chocolate chips and toffee bits for the topping. Fold the remaining chips and toffee into the batter until evenly distributed.

Scrape the batter into the prepared pan. Spread it into the corners and smooth the top into an even layer. Sprinkle the reserved chocolate chips and toffee bits over the surface.

Bake for approximately 30 minutes, until the surface is set and a toothpick inserted near the centre comes out with moist crumbs rather than wet batter. If needed, bake a little longer and check again. If the tester hits a melted chocolate chip, test another nearby spot.

Place the pan on a wire rack and let the brownies cool completely. Lift them out using the parchment overhang, transfer to a cutting board, and cut into 16 squares. Wipe the knife between cuts for neater edges.