Move over Little Debbie, there's a new oatmeal cream pie in town and boy oh boy is it good!
Preheat oven to 375°F. Line 3 large cookie sheets with parchment paper. Set aside.
In a medium bowl, stir together the flour, baking soda, salt, cinnamon and oats until well combined. Set aside.
In a large bowl, add butter and sugar. Stir by hand or beat with an electric mixer until creamy. Add the egg, vanilla and molasses. Stir until combined. Gradually add the dry ingredients. Mix until everything is completely incorporated.
Use a 1 1/2 tablespoon cookie scoop to scoop out 30 mounds of cookie dough. They will be dome-like because of the scoop. (Use a regular spoon if you don't have a cookie scoop and divide the dough equally into 30 pieces.) Place 10 dough "balls" per prepared cookie sheet. Bake for 10-12 minutes. If the cookies are dome like, use the bottom of a glass to flatten them slightly. Let cool.
Use a hand mixer to beat the shortening until creamy. Gradually add the powdered sugar and water. Once combined, beat on medium speed for about a minute. You can also do this by hand if you don't have a mixer.
Spread filling onto 15 cookies, then place the plain cookies on top. They are ready to serve.
Gluten Free Option: If you would like to make these gluten free, then substitute gluten free flour for the regular flour. And also make sure to get gluten free oats. Even though oats are naturally gluten free, it's best to check the package. I tested this recipe with gluten free flour and the cookies come out a little bit crispy but still taste great. You might have to press down the cookies with the bottom of a glass when they come out of the oven to make them more flat.