Preheat the oven to 325°F (165°C) and line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl, stir the melted butter, brown sugar, and granulated sugar together until well blended.
Add the egg yolk and vanilla, and stir until the mixture is creamy.
In a separate small bowl, whisk the flour, baking soda, cinnamon, and salt together.
Stir the dry ingredients into the wet mixture until no flour streaks remain.
Fold in the oats until they're evenly worked through the dough.
Scoop the dough into mounds with a medium cookie scoop or a heaping tablespoon, spacing them about 2 inches apart on the prepared baking sheet.
Bake for 8 to 10 minutes, until the edges are lightly golden and the centers still look soft. They finish setting as they cool.
Cool the cookies on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
Notes
Scoop the dough freeform: Scoop the dough into loose mounds instead of packing it into tight balls. When I tested this recipe, the freeform mounds held their shape better in the oven.Measure the flour correctly: Spoon the flour into a dry measuring cup and level it off. Scooping straight from the bag packs in extra flour and dries the cookies out.Take them out when the edges are golden: Pull the cookies at 8 to 10 minutes, when just the edges are lightly browned. They finish setting on the hot sheet as they cool.Let them firm up before moving: Cool the cookies on the sheet for 5 minutes before transferring them. They're soft right out of the oven and hold together better once they've set.