Preheat the oven to 350 degrees. Prep a 8×8 or a 9×9 pan by lining with parchment paper and spraying with cooking spray. Set aside.
In a separate mixing bowl, whisk together the flour, baking powder, and salt.
In a medium sized bowl, with a whisk, whisk the melted butter and sugar until light and fluffy, about 2 minutes
Whisk in the egg and egg yolk for 30 seconds and then add the vanilla extract Whisk just until combined.
With a rubber spatula, fold the flour, cornstarch, baking powder, salt, dark chocolate, and toffee pieces into the wet ingredients. Just mix until there are no more signs of flour. Don’t over mix.
Place the dough into the prepped pan and evenly distribute across the pan.
Garnish with a little extra pieces for chocolate and toffee.
Bake for 25-28 minutes. Mine took exactly 27 minutes.
When the bars are done, let them cool completely before cutting. Hint (top with a flaky salt for an extra delicious flavor)