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.