Step 1: Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides, and lightly grease.
Step 2: In a medium bowl, melt the unsalted butter. Let cool for 5-10 minutes.
Step 3: In a large bowl, whisk together the cooled melted butter, light brown sugar, and granulated sugar for 1-2 minutes until smooth.
Step 4: Add the eggs one at a time, whisking well after each. Stir in the pure vanilla extract.
Step 5: In a separate small bowl, whisk together the all-purpose flour, baking powder, and salt.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Step 7: Gently fold in optional chocolate chips or other mix-ins. Transfer batter to the prepared pan and spread evenly.
Step 8: Bake for 25-30 minutes, or until edges are golden and set, and a toothpick inserted into the center comes out with moist crumbs. The center should look slightly underdone.
Step 9: Let cool completely in the pan on a wire rack before lifting out and cutting into squares.