Prepare brownie mix according to package directions; stir in pecans. Spoon batter into a greased 13 x 9-inch baking pan. Bake at 350 degrees F for 25 minutes. Remove from oven and sprinkle miniature marshmallows over hot brownies.
Combine chocolate, milk and butter in a heavy saucepan. Cook over low heat until chocolate and butter melt, stirring often. Remove from heat. Transfer to a medium mixing bowl. Add confectioners’ sugar and vanilla extract. Beat at low speed with an electric mixer until smooth. If frosting is too stiff for spreading consistency, add milk, 1 tablespoon at a time, stirring until smooth. Spread over brownies. Cool in pan on a wire rack, and cut into bars.
