Heat oven to 350° F. Combine flour, cocoa, baking soda and salt in bowl; set
aside. Combine milk and vanilla in measuring cup; set aside. Beat egg and oil in
bowl of electric mixer until combined.
Add sugar to egg-oil mixture; beat until pale yellow, about 3 minutes. Add flour
and milk mixtures alternately to egg mixture, beginning and ending with flour
mixture.
Drop about 1 tablespoon of the batter onto greased cookie sheets, 2 inches
apart. Bake until top springs back lightly when touched, 5-7 minutes. Remove to
wire racks to cool.
For filling, combine marshmallow spread, confectioners' sugar, butter and
vanilla in bowl of electric mixer. Beat on medium speed until fluffy, about 2
minutes. Spread filling between 2 cake halves to make sandwich.
