Preparation

• Begin by melting the unsalted butter in a mixing bowl. You can do this in a small saucepan over low heat or in a microwave in short bursts. Allow it to cool slightly before incorporating the sugar.
• Add the light brown sugar to the melted butter. Use a whisk to combine them thoroughly until smooth with the brown sugar dissolved.
• Add the large egg and egg yolk to the butter and sugar mixture. Whisk together until fully combined, and the mixture becomes thicker and creamy.
• Introduce the vanilla extract and fine sea salt, stirring until well incorporated.
• Sprinkle the baking powder over the wet ingredients and mix briefly to distribute it evenly.
• Gradually add the all-purpose flour to the mixture, mixing just until no dry flour remains. Avoid overmixing.
• Fold in your chosen mix-ins until they are evenly distributed.
• Transfer the batter into a greased baking pan, spreading evenly with a spatula.
• Sprinkle a little flaky sea salt over the surface of the batter before baking.
• Place the pan in the preheated oven and bake until the edges are set and the center looks slightly underbaked, about 25 minutes.