First, preheat the oven to 350ºF and line a baking sheet with parchment paper.
Next, combine melted butter and brown sugar until there are no lumps (we used an electric mixer for this step, but you can use a hand whisk, too). Then, add maple syrup, egg, peanut butter, and vanilla and mix again.
Add salt, quick-cooking oats, cocoa powder, and baking soda and mix until everything is combined. Add in chocolate chunks and mix again.
Refrigerate dough for 15 minutes to set.
Scoop 2 heaping tablespoons of cookie dough into your hands and form it into a ball. Place it onto the cookie sheet and slightly press down to form a cookie shape. The batter will feel a bit wet, but it will be so soft and chewy when done baking!*
Repeat the process above until you've fit 12 cookies on the baking sheet.
Add more chocolate chunks to the top, if desired.
Bake cookies at 350ºF for 11-14 minutes or until they begin to crack on the top.
Remove cookies and let them cool on the baking sheet for a couple of moments before transferring them to a wire rack to continue cooling for at least 20 minutes to firm up. Sprinkle the cookies with flakey sea salt.