Preheat the oven to 350°F and line a baking sheet with parchment paper.

Combine melted butter and brown sugar until smooth. Add maple syrup, egg, peanut butter, and vanilla; mix well.

Incorporate salt, oats, cocoa powder, and baking soda; stir until combined. Fold in chocolate chunks.

Refrigerate the dough for 15 minutes to set.

Scoop out 2 heaping tablespoons of dough, form into balls, and place on the baking sheet. Slightly press down.

Repeat until the sheet is filled with 12 cookies. Add more chocolate chunks on top if desired.

Bake for 12 minutes or until edges are slightly cracked. Let cookies cool on the sheet briefly.

Transfer to a wire rack. While still warm, sprinkle with flaky sea salt. Cool for 20 minutes before serving.