Preheat oven to 325°F; grease an 8×8-inch baking pan, or coat with a non-stick cooking spray.
Combine flour, baking powder, baking soda and salt in large mixing bowl; set aside.
In a medium bowl, mix softened.
butter and sugar until smooth; stir in eggs and egg
yolk.
Add this mixture to the dry ingredients; add vanilla extract and buttermilk; stir well.
In a third bowl, use a pastry cutter to combine topping ingredients until it resembles crumbs.
Spoon about three quarters of the batter in the prepared pan, sprinkle about one quarter of the topping over the batter; spoon remaining batter onto this topping layer, then cover all with remaining topping.
Bake ~55 minutes, until a toothpick inserted in the center of cake comes out clean.
Remove cake from oven; let cool ~10 minutes; drizzle with icing, if desired; cut into 9 pieces.
