In a medium bowl, combine flour, baking soda, salt and cinnamon.
In a large bowl, cream together (beat until well mixed) butter and brown sugar. Add eggs and vanilla. Gradually add to creamed mixture. Stir in oats. Using a cookie scoop, or a spoon, drop dough by rounded spoonfuls onto Silpat lined or lightly greased baking sheet. Bake for 8-11 minutes. Let cool completely before filling.
To make filling, combine powdered sugar, vanilla, peppermint extract, and shortening until creamy. Fold in beaten egg whites. Spread filling between two cookies to create a sandwich.
Spread crushed peppermint candies on a small plate. Roll each filled sandwich cookie in crushed peppermint, or spoon crushed peppermint onto sides of sandwich cookies. Wrap each cookie individually in plastic wrap, and store cookie sandwiches in the fridge.
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