Step 1

Preheat the oven to 350°F. Line large baking sheets with parchment paper.

Step 2

In a medium bowl, whisk together the flour, salt, baking powder, baking soda, and cinnamon.

Step 3

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.

Step 4

Using a spoon or large spring-loaded scoop, drop 3-tablespoon sized balls of dough onto prepared baking sheets. Flatten slightly with the bottom of a measuring cup.

Step 5

Bake for about 12 to 14 minutes, or until the edges are slightly browned, rotating baking sheets halfway through. Let the cookies cool on baking sheets for 5 minutes before removing to wire racks to cool completely. Cookies can be stored in an airtight container at room temperature for up to 3 days.

Step 6

Beat all ingredients together with an electric mixer on medium-high speed until light, fluffy, and smooth, about 5 minutes.

Step 7

Spread or pipe the filling on half of your cookies and use the remaining cookies to sandwich. Serve or store in an airtight container in the fridge for up to 3 days. Let come to room temperature before serving.