Combine brown sugar, flour, and cinnamon in a mixing bowl.
Cut in until mixture resembles coarse crumbs. Set aside.
Combine softened margarine/butter, flour, cinnamon, sugar, and baking powder in a mixing bowl. Make a well in the center.
Beat egg, milk, and oil in a small bowl. Stir in apple. Pour into well. Stir until just moistened.
Fill standard muffin cups ¾ full with batter. Sprinkle streusel topping over the batter.
Bake at 400 °F (200 °C) for 15 to 20 minutes or until golden.