In a large bowl, whisk together the flour, oats, baking soda, cinnamon, and salt.
In a separate bowl, whisk the melted butter, brown sugar, granulated sugar, and molasses together until smooth. Add in the egg, egg yolk, and vanilla, whisk until completely incorporated.
Pour the wet mixture into the dry ingredients and stir until everything is well combined.
Let the dough rest for 5–10 minutes. This helps it firm up so it’s easier to shape.
While the cookie dough is sitting, preheat the oven to 350°F and line 3 to 4 baking sheets with parchment paper.
Scoop about 1 heaping tablespoon of dough (a cookie scoop works great if you have one). Roll each portion into a ball and place it on the parchment-lined baking sheet. Use your fingers to flatten each ball into a 2-inch circle, about ½ inch thick. The cookies spread quite a bit as they bake, so space them at least 2 inches apart - you’ll only fit about 8 cookies per large baking sheet.
Bake one sheet at a time for 9-12 minutes, until the edges are lightly browned and the centers look soft and a little underbaked, but not shiny or wet. They’ll continue cooking and firm up as they cool on the pan. Let the oatmeal cookies cool completely before filling.
Tip: For uniform cookies, while the cookies are still hot, use a large round cookie cutter or a small bowl to mold the edges of the cookie into circles. This will keep them all the same size and perfectly round, and it will also make the centers very soft and chewy after cooling.