Preheat your oven to 350°F (175°C).
Grease your 9x9-inch baking pan with a little butter or line it with parchment paper for easy removal later.
In a mixing bowl, combine the softened unsalted butter and firmly packed dark brown sugar. Use a spatula or hand mixer to cream them together until the mixture is light and fluffy, about 2-3 minutes.
Add the egg yolk and vanilla extract to the creamed mixture. Beat until well combined.
In a separate bowl, whisk together the all-purpose flour and salt. Gradually add the dry ingredients to the butter mixture, mixing until just combined. Be careful not to overmix!
In a microwave-safe bowl, combine the chopped bittersweet or semisweet chocolate with the heavy whipping cream. Heat in the microwave in 30-second intervals, stirring in between, until the chocolate is completely melted and smooth.
Fold the melted chocolate into the batter until fully incorporated. Then, gently fold in the toffee pieces, ensuring they are evenly distributed throughout the mixture.
Pour the batter into the prepared baking pan, spreading it evenly with your spatula. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Once baked, remove the pan from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a cooling rack to cool completely before slicing into wedges.