Cream margarine, and gradually add sugar, beating at medium speed of a mixer until well-blended. Add eggs, 1 at a time, beating well after each addition.
Combine flour, cocoa, baking powder, and salt; stir well. Add to creamed mixture, beating at low speed until blended. Stir in pecans and vanilla. Pour batter into a 13 x 9-inch baking dish coated with cooking spray. Bake at 325° for 16 minutes or just until set (not until toothpick tests clean, or cake will be overbaked). While cake bakes, prepare Chocolate Glaze; set aside.
Remove cake from oven; top with marshmallows. Bake 2 minutes or until marshmallows are soft. Remove from oven; drizzle with Chocolate Glaze, and let cool.