1. Stir together gingerbread cake mix, eggs, and 2 1/2 cups stout beer in a large bowl until combined. Pour batter evenly into 2 lightly greased 8-inch square pans.
2. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on a wire rack 10 minutes. Remove from pans, and let cool on wire rack 1 hour or until completely cool.
3. Beat softened butter at medium speed with an electric mixer until creamy. Gradually add powdered sugar and remaining 1/4 cup stout beer, beating until blended after each addition. Beat 1 minute or until light and fluffy.
4. Spread stout buttercream between layers and on top of cake. Garnish, if desired.
Note: For testing purposes only, we used Betty Crocker Gingerbread Cake & Cookie Mix. We used Terrapin Wake-n-Bake Coffee Oatmeal Imperial Stout at one testing and Guinness Extra Stout beer at another.