Amazing chewy, oatmeal cookies with maple brown sugar buttercream sandwiched between. It's like eating a warm, chewy bowl of oatmeal with all the fixings!
Preheat oven to 350°F and line a cookie sheet with parchment paper. In a medium mixing bowl mix together flour, salt, baking soda and cinnamon (optional), set aside.
Place softened butter in a mixer bowl and cream together with sugars until light and fluffy. About 2-3 minutes on medium-high, scraping down sides once or twice. Add eggs, one at a time and beat well after each addition, about 1 minute each. Add in vanilla, mix in.
Scooping about ½ cup of flour mixture at a time, mix on low speed until all flour is combined. Scraping down sides once, then add oats and mix until incorporated.
Using a medium sized scoop, scoop dough balls onto parchment lined baking sheets and refrigerate for 10-15 minutes prior to baking.
Bake for 10-12 minutes until edges are golden, cool on pan for 2-3 minutes then transfer to cooling rack to cool completely before adding frosting, if desired, sprinkle a little flaky sea salt.
FOR THE BUTTERCREAM
Place softened butter and brown sugar in the bowl of a mixer and beat on medium-high for 2-3 minutes, then let the mixture sit to "melt" into the butter for about 10-15 minutes. If still "gritty" beat again for another 1-2 minutes, then let sit for a couple more.
Add powdered sugar and mix on low until combined. With mixer on low, add half and half, maple syrup, vanilla and salt, mix until combined, then increase speed to medium high for 3-5 minutes until light, fluffy and creamy. If too thick, add a teaspoon or two more of cream. If gritty in any way, keep beating on high occasionally scraping sides of bowl for up to 7-8 minutes, eventually it will break the sugar crystals down.
Scoop a generous portion of buttercream onto the bottom of an oatmeal cookie, finding a similar sized cooked, place bottom side down on top of buttercream, pressing lightly to smush frosting to edges. Wrap each sandwich cookie in plastic wrap.
Store cookies in airtight container or baggie for up to 3 days in refrigerator or up to 2 months (if you are lucky) in the freezer. Allow to come to room temperature for an hour before serving.