Lightly grease and flour an 8 inch, heat-resistant, non-metallic cake dish.
Preheat a conventional broiler.
Cream butter and the 3/4 cup sugar together in a large bowl until fluffy.
Beat in egg.
Sift the flour, baking powder and salt together.
Add flour mixture and milk to the sugar mixture alternately, beginning and ending with flour mixture.
Pour into prepared cake dish.
Spread evenly.
Combine the 3/4 cup dark brown sugar and cinnamon in a small bowl and sprinkle over the batter.
Heat, uncovered, on roast for 8 to 9 minutes.
Insert a wooden pick to test for doneness.
Place cake in a conventional oven 4 inches away from broiler element for a few minutes or until topping is browned.