Working with a stand mixer, preferably fitted with a paddle attachment, or a hand mixer in a large bowl, beat the butter at medium speed until smooth, about 3 minutes. Add the sugars and beat for another three minutes or until the mixture is light and creamy. Beat in the vanilla and turn off the mixer. Add all the dry ingredients, cover the stand mixer with a kitchen towel (so you and your kitchen don’t get showered in flour) and pulse the mixer on and off at low speed about 5 times- at which point a peek at the bowl should reveal that it’s safe to turn the mixer to low and mix, uncovered, just until the dry ingredients are almost incorporated. Add the chopped chocolate and mix only until the dry ingredients disappear. If the chocolate isn’t evenly mixed, finish the job by hand with a spatula. You’ll have a very heavy, very sticky dough. Scrape the dough into the buttered pan and, with the spatula and your fingertips, cajole it into a thin, even layer.
Bake for 20 to 22 minutes, or until the base is bubbly – so bubbly that you can almost hear it percolating – and puckery. It will look as though it is struggling to pull away from the side of the pan. Transfer the pan to a rack and turn off the oven.
To make the topping: Scatter the chocolate evenly over the top of the hot base and pop the pan back into the oven for 2 to 3 minutes, until the chocolate is soft.. Remove from oven and immediately spread chocolate over bars, using offset spatula or the back of a spoon. Sprinkle the toffee bits over the chocolate and press them down lightly with your fingertips. Place the baking pan on a rack to cool to room temperature.
If, by the time the bars are cool, the chocolate has not set, refrigerate them briefly to firm the chocolate.
Carefully lift out of the pan, using foil edges as handles, and transfer to a cutting board. Trim the edges if they seem a bit thick. Cut about 20 bars in total, or more, depending on how large you prefer the squares to be.