Preheat your oven to 350°F (175°C) to ensure it's ready for baking.
In a large mixing bowl, melt the butter in the microwave or on the stovetop. Allow it to cool slightly before proceeding to the next step.
Add the packed dark brown sugar to the melted butter and whisk until well combined.
Lightly beat the eggs in a separate bowl, then add them along with the vanilla extract to the sugar mixture. Whisk until fully incorporated.
In the same bowl, add the all-purpose flour, baking powder, baking soda, and salt. Use a rubber spatula to gently fold the dry ingredients into the wet mixture until just combined.
Gently fold in the chocolate chips and toffee pieces.
Line your baking pan with parchment paper or simply grease it with butter or cooking spray.
Pour the blondie batter into the prepared baking pan, spreading it evenly with your spatula.
Sprinkle a little flaked sea salt on top of the batter.
Bake in the preheated oven for 20-25 minutes or until the edges are golden brown.
Allow the blondies to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. Once cooled, slice into squares and enjoy!