Preheat the oven to 350 and grease a 9x9 or 8x8 baking tin. Combine flour, salt, baking soda, and baking powder into a medium bowl and set aside.
In a large bowl, mix together the butter and sugar until creamy. Then add the vanilla, egg, and buttermilk. Mix until well combined.
Add dry ingredients to wet ingredients and mix until just combined.
Spread cake mixture into greased pan.
Make the crumble: Cut butter into the flour and sugar with either a fork or rub between your fingers. The goal is to create a crumble texture so it shouldn't be super fine crumbs. It tastes delicious if there are bigger chunks of butter in the crumble.
Sprinkle crumble on top of the cake mixture in the baking tin.
Bake for 45-50 minutes or until a toothpick comes out clean. Let cool for 10-15 minutes. Enjoy!