Heat oven to 350 degrees F; coat 9-inch square baking pan with cooking spray.

For topping, in medium bowl, blend brown sugar, flour, cinnamon and salt.

With pastry blender, cut butter into flour mixture until crumbs form; chill.

For cake, (if not using buttermilk) pour vinegar into a measuring cup; add enough milk to equal 1 cup.

Set aside.

In medium bow, whisk together flour, baking powder, baking soda and salt.

In large bowl, beat butter, sugar and vanilla until fluffy, 2 minutes.

Add eggs, one at a time, beating well after each.

Working in batches, beat in milk mixture alternately with flour mixture.

Scrape batter into pan.

Sprinkle with topping.

Bake at 350 degrees for 1 hour, until a toothpick inserted into cake comes out clean.

Cool in pan on wire rack.

Dust with confectioners' sugar; cut in squares.

If you don't own a powdered sugar sifter, use a wire-mesh container or strainger to dust top of cake.