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.
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.
For the cake:
Preheat the oven to 350f/175c. Grease 9x13 inch baking pan
In a medium mixing bowl mix together the flour, baking soda, baking powder, and salt.
Beat the softened butter in a large bowl until light.
Add sugar and beat until light and fluffy.
Add eggs 1 at a time, beating until well blended after each addition.
Add buttermilk and vanilla, beat just until blended.
Add flour mixture in 3 additions, beating just until blended after each addition.
Transfer cake batter to prepared baking pan (the batter will be thick); spreading it evenly over the pan.
Hand-squeeze small handfuls of topping together and crumble clumps evenly over cake batter, covering with a thick layer of topping.
Bake 45 minutes to an hour or until the tester comes out clean and topping is deep golden brown and a little crisp.
Cool cake in dish on rack at least 30 minutes. Dust with powdered sugar and cut into squares to serve.
Make the topping first. Leaving the batter to sit before baking may cause problems with rising, causing the topping to sink into the cake.
Use good quality unbleached flour (with4-5g of protein) for the best result. Lower quality flour may not have the stability to hold the topping.
Don't overmix the batter once the flour is added. Overmixing can cause a tough, dense cake.
Be sure to cool the cake completely before dusting with the sugar. A warm cake will melt the sugar.
If you don't have buttermilk on hand don't panic, you can make your own. 1 cup of milk plus 1 tbsp of white vinegar or lemon juice is a great rule of thumb.For this recipe: 1 1/4 cup of milk plus 1 1/4 tablespoons of white vinegar or lemon juice.
Can you freeze Crumb Cake?
Crumb Cake Freezes really well. Just wrap well and freeze for up to 3 months. Defrost at room temperature overnight.