Preheat oven to 325°. Stir together 2 cups flour, 1/2 cup sugar, brown sugar, cinnamon, and melted butter in a bowl. Set aside.
Beat softened butter and remaining 1/2 cup sugar at medium speed with an electric mixer until fluffy. Add egg and vanilla, beating until blended.
Combine baking soda, salt, and remaining 1 1/2 cups flour; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition.
Spoon batter into a greased and floured 8-inch square cake pan, and sprinkle with reserved topping mixture.
Bake 30 to 35 minutes or until a wooden pick inserted in the center comes out clean. Cool; dust with powdered sugar, and cut into squares.