Try shortening the baking time slightly. Take the cookies out when the edges are set and the centers still look soft and a little underdone, then leave them on the baking sheet for a few minutes to finish setting. Waiting until the centers look fully baked often leaves you with crisp cookies once they cool.

Resting the dough and adding more brown sugar make sense: chewy cookies need to hold onto moisture. A few other things can help:

- Use about 2 parts brown sugar to 1 part white sugar. Brown sugar helps the cookies retain moisture.
- Chill the dough overnight so the flour has time to absorb moisture. Chilling also limits spreading, keeping the cookies thicker.
- Shape the dough into tall mounds. Thicker cookies tend to have chewy centers, while thin ones dry out quickly.
- Try an extra egg yolk for richness and a softer, chewier texture.
- Once the cookies are cool, store them in an airtight container. Left exposed, they dry out and become crisp.

A little maple syrup can help retain moisture, as you found, though too much changes the dough’s consistency. Go easy on extra butter, too: it can make the cookies spread more and give them crisper edges.