Preheat oven to 350°F. Wash and dry blackberries gently, slicing in half if large. Set aside.
Cut butter into chunks, keeping 2 tablespoons separate from the 1/2 cup.
Place flour, sugar, salt, baking soda, and baking powder into the bowl of a food processor. Pulse to combine. Add in 1/2 cup of butter. Pulse until mixture resembles coarse crumbs no larger than peas.*
Dump flour mixture into a large bowl. Return 1 cup to the food processor, add the remaining butter and pecans. Pulse until nuts break down and you have a coarse crumb mixture. Refrigerate crumb mixture.
Add buttermilk, eggs, and vanilla to flour mixture. Stir until incorporated. The butter will remain chunky, this is what you want. Just stir until there are no dry spots or obvious spots of egg.
Pour into a 9x13 baking dish. Top with blackberries, then with the crumb mixture. Bake for 35 minutes or until a toothpick comes out clean.
Cool at least 10 minutes, cut and enjoy! Store in an airtight container or covered with plastic wrap for 3 days.*
* If you don't have buttermilk, place 1 tablespoon of white vinegar or lemon juice in a liquid measuring cup. Fill with milk to the 1 cup measurement. Stir and let sit. Do this at the beginning of the recipe and it will be ready by the time you need it. *This is possible to do by hand. Cut the 1/2 cup of butter into the flour sugar mix by hand, remove one cup and cut the remaining butter into that. Chop the pecans by hand and mix to make the crumb topping.*This cake is great day 2! Bake ahead for a lovely morning treat.