Preheat oven to 350°F. Prepare Treatwiches Mini Cake Pan with vegetable pan spray.
In medium bowl combine flour, cocoa, baking powder, baking soda and salt.
In large bowl, beat butter and sugar with electric mixer until light and fluffy.
Add egg and vanilla; mix well. Add flour mixture and beat until just moistened.
Add milk; beat until just combined. Fill cavities 2/3 full with batter.
Bake 9-11 minutes or until toothpick inserted comes out clean. Cool in pan 5 minutes. Remove from pan; cool completely.
In large bowl, beat butter and sugar with electric mixer until well combined, scraping down bottom and sides of bowl as necessary. Add vanilla and milk and beat until light and fluffy. Spread filling onto cake and sandwich with another.