Preheat the oven to 350 F. Line two large baking sheets with parchment paper. Set aside.
In a small bowl, melt the butter and cool in the fridge for about 10 minutes.
In a separate bowl, whisk together the flour, oats, baking soda, ground cinnamon and nutmeg, and salt until well combined.
Whisk the cooled melted butter with both sugars together in a separate large bowl until well combined. Whisk in the egg, egg yolk, molasses vanilla extract until smooth and well combined.
Stir the dry ingredients into the wet a little at a time until just combined. Using a medium cookie scoop, scoop the oatmeal cookie dough and place the dough balls onto the prepared baking sheets. 14 per sheet.
Bake at 350 F. for only 12 minutes or until the edges are set and the centers are slightly puffed. Cool cookies on the baking sheet. Let them cool completely before icing.