1. Preheat oven to 350 degrees F. Grease 9 x 9 x 2-inch-square baking pan.Prepare Crumbly Topping:
2. Combine flour, sugar and cinnamon in large bowl. Work butter in with your fingertips until no traces of butter remain. (You may also pulse the dry ingredients in processor to combine. Then pulse in the butter.)Prepare Cake:
3. Combine flour, baking powder, baking soda, sugar and salt in medium-size bowl. In a separate bowl, whisk together buttermilk, butter, eggs and vanilla. Add to flour mixture and stir until just combined. Spread dough in prepared pan. Cover with crumbly topping.
4. Bake in 350 degrees F oven for 40 to 45 minutes or until top is slightly cracked, crumbs are brown and cake tester inserted in center comes out clean. Transfer pan to wire rack to cool. Sprinkle with powdered sugar. Cut into 16 squares.