Mix margarine, sugar, milk and egg.
Stir in flour, baking powder and salt.
Spoon into 6 paper-lined plastic muffin cups or custard or coffee cups, filling 1/2 full.
Spoon rounded teaspoonful Streusel Topping onto each muffin.
Microwave 1 1/2 minutes.
Rotate or rearrange cups and microwave until no longer doughy, 30 seconds to 1 minute.
Immediately remove from cups.
Repeat with remaining batter and topping.