Line an 8×8-inch baking pan with parchment paper. Leave a little overhang on the sides so you can lift the bars out easily later.
In a mixing bowl, beat together the softened butter, brown sugar, and granulated sugar. Mix until creamy and smooth. If you do this by hand, it takes a minute or two
Crack in the egg and pour in the vanilla extract. Mix again until everything blends together.
Sprinkle the flour, baking soda, and salt over the wet mixture. Stir gently until everything comes together. Avoid overmixing — once the flour disappears, stop.
Add the chocolate chips (and chunks if you’re using them). Stir just enough to distribute them.
Scoop the dough into your prepared pan. Use your fingers or a spatula to press it gently into an even layer.
Bake at 175°C (350°F) for 18–22 minutes. The edges should look lightly golden, and the center should look slightly soft, even a bit underbaked.
Let the bars rest for at least 20–25 minutes before cutting. Warm slices taste amazing but may crumble a little (which honestly, is part of the charm). Fully cooled bars slice into perfect squares.
