Step 1: Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
Step 2: In a large microwave-safe bowl, melt the unsalted butter.
Step 3: Add the light brown sugar and granulated sugar to the melted butter. Whisk until well combined and smooth.
Step 4: Whisk in the egg and egg yolk until fully incorporated, about 1 minute.
Step 5: Stir in the vanilla extract until combined.
Step 6: In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt.
Step 7: Gradually add the dry ingredients to the wet ingredients, mixing with a wooden spoon or spatula until just combined. Be careful not to overmix.
Step 8: Gently fold in your chosen mix-ins (chocolate chips, nuts, etc.).
Step 9: Drop rounded tablespoons of dough onto the prepared baking sheets, leaving about 2 inches between each cookie.
Step 10: Bake for 10-12 minutes, or until the edges are set and lightly golden, but the centers still look a little soft. Do not overbake.
Step 11: Remove from the oven and let the cookies cool on the baking sheets for 5-10 minutes before transferring them to a wire rack to cool completely.