Homemade Oatmeal Cream Pies

Preheat the oven to 350°F. Line large baking sheets with parchment paper.
In a medium bowl, whisk together the flour, salt, baking powder, baking soda, and cinnamon. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy, about 2 to 3 minutes. Beat in the eggs, egg yolk, and vanilla, one at a time, beating well after each addition. Slowly beat in the flour mixture until just combined. Stir in the oats with a rubber spatula.
Using a spoon or cookie scoop, drop balls of dough onto prepared baking sheets. For smaller cookies similar in size to store-bought Oatmeal Cream Pies, use a medium cookie scoop (1 1/2 tablespoons). For larger bakery-style cookies, use a large cookie scoop (3 tablespoons). Space cookies 6 per half-sheet pan and flatten slightly with the palm of your hand.
Bake smaller cookies for about 10 minutes or larger cookies for about 12 minutes, until the edges are lightly browned and the centers have just set. Cookies will be slightly delicate while warm. Let them cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely. Store cookies in an airtight container at room temperature for up to 3 days.

Mix all ingredients together with an electric mixer on low until combined, then beat on medium-high speed until light, fluffy, and smooth, about 5 minutes.
Spread or pipe the filling on half of your cookies and use the remaining cookies to sandwich. Serve or store in an airtight container at room temperature for up to 1 day or in the fridge for up to 2 days. Let come to room temperature before serving.