Gluten-Free Lemon Poppy Seed Sandwich Cookies

Preheat oven to 350ºF and line baking sheets (as many as you have) with parchment paper.

Beat butter with a hand or stand mixer for 30 seconds. Add cake mix and mix until combined. Beat in egg, vanilla, lemon zest, and poppy seeds until well combined.

Scoop cookie dough by the teaspoon full and roll into a ball. Place balls about 1 inch apart on cookie sheets and flatten a little with the back of your teaspoon measuring spoon.

Bake 7 or 8 minutes. Cool on pan for 2 minutes. Then transfer to cooling rack and cool completely.

For frosting, beat butter and shortening with hand or stand mixer for 30 seconds. Beat in 1 cup powdered sugar. Beat in vanilla. Beat in remaining powdered sugar.

Beat milk and scrape down bowl. Beat for 3 minutes until light and fluffy.

Place a small dollop (about 1 1/2 teaspoons) onto one cookie. Put another cookie on top and gently push together so frosting spreads evenly. Repeat with remaining cookies and frosting.