Step 1: Preheat the Oven - Preheat your oven to 350°F (175°C) to ensure even baking.
Step 2: Melt the Butter - Melt 2 tablespoons of unsalted butter in a mixing bowl and let it cool slightly.
Step 3: Combine Sugar and Butter - Stir 1/4 cup of packed brown sugar into the melted butter until creamy.
Step 4: Add Egg Yolk and Vanilla - Mix in 1 large egg yolk and 1/2 teaspoon of vanilla extract.
Step 5: Mix Dry Ingredients - In a separate bowl, whisk together 1/4 teaspoon of baking powder, 1/8 teaspoon of salt, and 1/3 cup of all-purpose flour, then combine with the wet mixture.
Step 6: Fold in Mix-Ins - Gently fold in your choice of mix-ins.
Step 7: Pour and Spread - Pour the batter into a greased skillet and spread it evenly.
Step 8: Bake - Bake for 15-18 minutes until the edges are golden brown and the center is soft.
Step 9: Cool and Serve - Let cool for a few minutes before slicing and enjoy warm!