Melt 12 ounce package chocolate chips in medium saucepan over medium-low heat, stirring occasionally; cool.
In mixer bowl, beat chocolate mixture, eggs and sugar. Blend in flour, baking powder, salt and vanilla. Spoon into greased 9x13" baking pan.
Bake in a preheated 375 degree oven 25 minutes. Remove from oven, sprinkle with marshmallows, pecans and remaining chocolate chips, in that order. Continue baking until golden (15-20 minutes). Cool. Cut with knife dipped in warm water.