Step 1

Mix the brown sugar, cinnamon, and salt together in a large bowl. Stir in the melted butter, then gently mix in the flour, being careful not to overmix (you just want to moisten the flour and mix until there is no visible dryness). Keep the mixture in large clumps. Set aside.

Step 2

Squeeze a handful of the topping into your hand, if it doesn't clump together, add a little more melted butter until it does. Set aside.

Step 3

Preheat the oven to 350f/175c.  Grease 9x13 inch baking pan

Step 4

In a medium mixing bowl mix together the flour, baking soda, baking powder, and salt.

Step 5

Beat the softened butter in a large bowl until light.

Step 6

Add sugar and beat until light and fluffy.

Step 7

Add eggs 1 at a time, beating until well blended after each addition.

Step 8

Add buttermilk and vanilla, beat just until blended.

Step 9

Add flour mixture in 3 additions, beating just until blended after each addition.

Step 10

Transfer cake batter to prepared baking pan (the batter will be thick); spreading it evenly over the pan.

Step 11

Hand-squeeze small handfuls of topping together and crumble clumps evenly over cake batter, covering with a thick layer of topping.

Step 12

Bake 45 minutes to an hour or until the tester comes out clean and topping is deep golden brown and a little crisp.

Step 13

Cool cake in dish on rack at least 30 minutes.  Dust with powdered sugar and cut into squares to serve.