• Whisk the flour, salt, spices and baking soda together in medium bowl; set aside.
• Process the oats in food processor until finely ground, about 1 minute.
• Whisk the sugars, oil, melted butter, and molasses in large bowl until smooth. Whisk in the egg, egg yolk and vanilla until smooth.
• Using a wood spoon or sturdy spatula, stir in the flour mixture until evenly combined and no dry flour remains. Stir in the ground oats until evenly distributed. Once the dough begins to stiffen, you can mix with your hands if needed until the dough is evenly combined. It is a fairly stiff dough.
• Cover and set the dough aside to rest for 30 minutes at room temperature.
• Preheat your oven to 400℉ and place a rack in the middle position.
• Divide dough into twenty-four equal sized portions (about 35-gram) or 1½ tablespoons each (3/4 ounce cookie scoop).
• Roll dough to form balls. Evenly space dough balls on prepared sheets, 8 balls per sheet. Using your hand or bottom of a glass, slightly flatten the balls to approximately 2 inch-wide.
  
• Bake one sheet at a time, until the tops start to crackle around the edges, about 6 to 7 minutes. They will puff up a little bit and then deflate and look crackled as soon as you take them out of the oven. The cookies will barely have any color and should seem a little underdone when you pull them from the oven. This is what you want, they will continue to bake as they cool on the baking sheet for 5 minutes. This ensures that the cookies remain chewy once they have cooled.
  
• Let the cookies cool on the baking sheet for 5 minutes. Using a spatula, transfer the cookies to a wire rack to cool completely, about 30 minutes.
  

Make the filling

• Mix all of the filling ingredients for 10 minutes on low speed in a stand mixer or with a hand mixer. Stop and scrape the bowl down a few times.
• Spread 1 – 1 ½ tablespoons of the filling on the bottoms (flat sides) of 12 cookies, I like to use the same ¾ ounce scoop that I use for the cookies. Top with remaining cookies to form sandwiches.
• Oatmeal cream pies can be stored in an airtight container at room temperature for up to 3 days or they can be frozen for longer storage.