For chewy chocolate chip cookies, the key is to **retain moisture and avoid overbaking**. Your results with resting the dough and increasing the brown sugar make sense.

- **Use more brown sugar than white sugar.** Try about 2 parts brown sugar to 1 part white sugar. Brown sugar helps the cookies retain moisture.
- **Chill the dough overnight.** This gives the flour time to absorb moisture and limits spreading, so the cookies stay thicker.
- **Bake just until the edges are set.** The centers should still look soft and slightly underdone when you remove them. Leave them on the baking sheet for a few minutes to finish setting.
- **Make thicker cookies.** Tall dough mounds tend to produce chewy centers; thin cookies dry out quickly.
- **Try an extra egg yolk.** It can add richness and a softer, chewier texture.
- **Store them airtight once cool.** Leaving them exposed lets them dry out and become crisp.

A little maple syrup can help retain moisture, as you found, but too much changes the dough’s consistency. Be cautious with extra butter: it can increase spreading and make the edges crisper.

If you change only one more thing, **shorten the baking time slightly**—waiting until the centers look fully baked often produces crisp cookies once they cool.