Cake:

1 1/2 cups all-purpose flour

2 1/2 teaspoons baking powder

1/2 teaspoon salt

1 egg, beaten

3/4 cup sugar

1/3 cup salted butter, melted

1/2 cup milk

1 teaspoon vanilla extract

Topping:

1/2 cup brown sugar

1/4 cup sifted all-purpose flour

1/4 cup salted butter, room temperature

1 teaspoon cinnamon

Heat the oven to 325° F. Butter and flour an 8-inch or 9-inch square or round layer cake pan.  In a small mixing bowl 1/2 cup of brown sugar with 1/4 cup of all-purpose flour, and the cinnamon. Stir to blend and then work in the 1/4 cup of softened butter with a fork until the mixture is crumbly.  Set the streusel aside.  In a medium bowl combine the 1 1/2 cups of all-purpose flour with the baking powder and salt; blend thoroughly and set aside.  In a mixing bowl beat the egg lightly and then beat in the sugar and the 1/3 cup of melted butter. Add milk and vanilla and blend well. Stir in flour mixture and mix until well blended.  Spread the batter in the prepared baking pan.  Sprinkle the brown sugar streusel topping mixture evenly over batter.  Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.