Melt margarine and chocolate in top of double boiler; set aside. Combine eggs and sugar, beating well. Add flour and baking powder which have been sifted together, mixing well. Add chocolate mixture, pecans and vanilla. Pour into 2 greased 9x13 pans and bake at 325 for 20 minutes. Remove brownies from oven and cover top with marshmallows. Return to oven for 5 minutes or just until marshmallows melt. Ice brownies while warm.
For Icing: combine sugar, milk, margarine and chocolate; cook over medium heat until soft ball stage (236 degrees). Add powdered sugar and vanilla. Beat until smooth. Spread over brownies while warm. Cut into squares when cool.