Preheat your oven to 350°F (175°C).
In a mixing bowl, melt 2 tablespoons of unsalted butter in the microwave or on the stovetop. Allow it to cool slightly before proceeding.
Add 1/4 cup of packed brown sugar to the melted butter. Whisk them together until the mixture is smooth and creamy.
Stir in 1 large egg yolk and 1/4 teaspoon of vanilla extract. Mix until well combined.
To the wet mixture, add 1/4 teaspoon of baking powder, 1/8 teaspoon of salt, and 1/3 cup of all-purpose flour. Stir gently until just combined.
Gently fold in your choice of mix-ins.
Pour the batter into your skillet, spreading it evenly. Bake for 10-12 minutes, or until the edges are golden.
Let the blondie cool for a few minutes in the skillet before slicing it into two pieces.
