In a saucepan over medium heat, melt the unsalted butter until it starts to foam. Stir continuously until it turns golden brown and develops a nutty aroma, taking care not to burn it.
Remove the browned butter from heat and let it cool slightly before whisking in the brown sugar until fully combined.
Once cooled, mix in the eggs one at a time, followed by the vanilla extract until everything is smooth and well incorporated.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually fold this mixture into your wet ingredients until just combined.
Gently fold in the toffee bits into the batter, ensuring even distribution throughout.
Pour the batter into a prepared baking pan and spread evenly before baking at 350°F (175°C) for about 25-30 minutes or until golden brown on top. Allow cooling completely before cutting into squares and serving.