In the bowl of a stand mixer fitted with the paddle attachment, beat the butter at medium speed until creamy. Gradually add the confectioners’ sugar and milk, beating until buttercream reaches a spreadable consistency. Add almond extract and beat until combined.

Transfer the buttercream to a piping bag fitted with a large star piping tip or a plastic bag with a corner snipped off. Pipe the buttercream in a swirl onto the flat sides of half the cookies. Sandwich with remaining cookies.