1/2cupgranulated sugarI recommend vanilla sugar, if you have it, for a little extra oomph
1large cage free eggroom temp
3/4teaspoonfine sea salt
1 1/2cupsall-purpose flour
3cupsold fashioned rolled oatsnot the quick cooking kind!
Combine filling ingredients in the bowl of a stand mixer fitted with the paddle attachment. Beat until you get a creme the consistency desired. Set aside.
Preheat oven to 375 degrees Fahrenheit. Line 2 rimmed cookie sheets with parchment paper and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together butter and sugars. Add the egg and vanilla and mix well. Sprinkle the baking soda, sea salt, and cinnamon around inside the bowl and while the mixer is running on slow, gradually add the flour. Once fully incorporated, add the rolled oats and mix until just incorporated.
Use a #60 cookie scoop for uniformly-sized and -shaped cookies. Drop on prepared cookie sheet and bake for about 10 minutes--be careful not to over-bake. Let sit on cookie sheet for 2-3 minutes before removing to wire racks to cool completely.
Take one cookie and smear creme filling on the flat bottom. Attach the flat bottom of another cookie to the filling to create a sandwich. Wrap individually or put in an air-tight container for up to 3 days at room temperature. They do improve with age so you may want to make them a few hours (minimum) to a day ahead of when you want to serve them.
Recipe was updated on 02/21/2016 to adjust the salt amounts.