Topping 

1/2 cup brown sugar 
1/4 cup sifted all-purpose flour (sift before measuring) 
1/4 cup butter, room temperature 
1 teaspoon cinnamon 

Cake 

1 1/2 cups sifted all-purpose flour (sift before measuring) 
2 1/2 teaspoons baking powder 
1/2 teaspoon salt 
1 egg, beaten 
3/4 cup sugar 
1/3 cup melted butter 
1/2 cup milk 
1 teaspoon vanilla extract 

Topping. In small mixing bowl, combine topping ingredients. Blend with fork until crumbly. Set aside.

Sift 1 1/2 cups sifted flour with baking powder and salt into a bowl. In a medium bowl, beat together beaten egg and 3/4 cup sugar and 1/3 cup melted butter. Add milk and vanilla. Stir in flour mixture and mix well. 

Pour batter into a greased and floured 8-inch square or 9-inch layer-cake pan. Sprinkle topping crumb mixture evenly over batter. Bake at 375° for 25 to 30 minutes, or until cake tests done. Partially cool in pan on wire rack. Cut coffee cake into squares while still warm.