Preheat the oven to 350°F (175°C).
Line a 9x13 inch baking pan with parchment paper, leaving overhang for easy removal.
Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
Cream softened butter with granulated and brown sugar in a large bowl until light and fluffy.
Add eggs and vanilla extract to the butter mixture, mixing until fully combined.
Gradually mix the dry ingredients into the wet mixture until just combined.
Fold in chocolate chips, nuts if using, and any other desired mix-ins evenly.
Spread the batter evenly in the prepared pan, smoothing the top.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Allow the bars to cool in the pan for 10 minutes, then lift them out using the parchment paper and cool completely on a wire rack.
Cut into squares or rectangles and serve.