Start by melting the unsalted butter in a medium saucepan over medium heat. Stir frequently until the butter turns golden brown and develops a nutty aroma, about 5-7 minutes. Be careful not to burn it!
Remove the saucepan from heat and let the butter cool slightly. In a mixing bowl, combine the brown sugar and granulated sugar. Pour the warm brown butter over the sugars and mix until well combined.
Add the large egg and egg yolk to the mixture, followed by the vanilla extract. Whisk until smooth and creamy.
In another bowl, whisk together the all-purpose flour, salt, and any additional flavors you may want to add. Slowly add the dry mixture to the wet ingredients, stirring gently.
Once the batter is smooth, fold in the chopped walnuts and toffee bits until evenly distributed.
Preheat your oven to 350°F (175°C). Line your 9x9-inch baking pan with parchment paper. Pour the batter into the pan and spread it evenly. Bake for 20-25 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the blondies from the oven and let them cool in the pan for about 10 minutes. Then, use the parchment overhang to lift them out and let them cool completely on a wire rack before cutting into squares.