01 - Preheat oven to 350°F. Grease and line an 8x8-inch baking pan with parchment paper.
02 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
03 - In a separate bowl, whisk melted butter, milk, eggs, and vanilla extract until combined.
04 - Add wet ingredients to dry ingredients; stir gently until just combined, avoiding overmixing.
05 - Fold in chocolate chips or berries if using.
06 - Pour batter into prepared pan, smoothing the top evenly.
07 - Bake for 25 to 28 minutes until the top is golden and a toothpick inserted into the center comes out clean.
08 - Allow to cool in the pan for 10 minutes, then remove and transfer to a wire rack to cool completely.
09 - Cut into squares and serve warm or at room temperature.