Cream butter and sugar until fluffy.
Beat in eggs and vanilla.
Sift dry ingredients together and stir into egg mixture.
Spread batter in a greased and floured 8" by 12" pan. Bake in a 350 degree oven for 25-30 minutes.
Remove from oven and sprinkle marshmallow halves over the top. Return to oven for 3-4 minutes.