Preheat oven to 350 degrees F and line a cookie sheet with parchment paper.

Whisk together the oats, flour, powder, and soda in a bowl and set aside.

In a stand mixer with the paddle attachment, beat the butter and both sugars together until light and fluffy.

Add in the dry ingredients and beat until combined with the butter and sugars – dough will be crumbly.

With the stand mixer on medium-low, pour in the almond milk. Beat only until a dough has formed – about 45 seconds to 1 minute.

Scoop and roll into 1 1/2 tablespoon balls and place on the prepared baking sheet about 2 inches apart. Lightly press the dough down with the palms of your hand. This helps get a thinner cookie similar to traditional cream pies.

Bake for 10 minutes or until lightly browned around the edges. They'll look a little underdone in the middle but they'll set up as they cool. Let cool completely before frosting.

Make the frosting by creaming together the butter in a stand mixer for 1 minute. With the stand mixer running, slowly add in the sugar until it's all incorporated. Add in milk, cinnamon, and vanilla and beat for 2-3 minutes or until the frosting is thicken and creamy. I like mine to be a bit thicker since it's going inside of cookies but you can add a little more milk as desired.

Once the cookies are cool, evenly divide the frosting between HALF of the cookies and spread it our so that there's about half an inch clean around the edges. Top each frosted cooking with a clean cookie to create a sandwich. Squeeze together lightly to press the frosting to the edges.

Enjoy immediately OR store in the fridge to let the frosting firm up a bit. This will make it so the frosting doesn't spill out the sides when you bit into the cookies.