Gather the ingredients including unsalted butter, light brown sugar, eggs, vanilla extract, all-purpose flour, baking powder, salt, and optional add-ins like chocolate chips, nuts, or dried fruit.
Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper or lightly grease it.
In a saucepan or microwave-safe bowl, melt the unsalted butter over low heat or in short bursts in the microwave until fully melted and smooth. Allow it to cool slightly.
In a large mixing bowl, whisk together the melted butter and light brown sugar until well combined and smooth.
Beat in the eggs, one at a time, then stir in the vanilla extract until smooth and creamy.
Sift in the all-purpose flour, baking powder, and salt into the wet ingredients. Use a spatula or wooden spoon to gently fold the dry ingredients into the wet ingredients until just combined. Be careful not to overmix.
If desired, fold in chocolate chips, nuts, or any other add-ins of your choice until evenly distributed throughout the batter.
Pour the blondie batter into the prepared baking pan and spread it out evenly with a spatula.
Bake in the preheated oven for 25 to 30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out with moist crumbs. Be careful not to overbake.
Allow the blondies to cool in the pan for at least 10 minutes, then lift them out using the parchment paper and transfer to a wire rack to cool completely. Once cooled, slice into squares and enjoy!