In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg and root beer concentrate. Combine the flour, salt and baking soda; gradually add to creamed mixture and mix well.
Shape dough into 3/4-in. balls. Place 2 in. apart on ungreased baking sheets. Bake at 375° for 6-8 minutes or until lightly browned. Remove to wire racks to cool completely.
In a small bowl, beat the filling ingredients until smooth. Spread on the bottoms of half of the cookies; top with remaining cookies.
To Make Ahead: Package cookies in an airtight container, separating layers with waxed paper and freeze for up to 1 month. Thaw in a single layer before filling.
Editor's Note
This recipe was tested with McCormick root beer concentrate.
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