Preheat your oven to 350°F / 175°C. Oil or line a muffin baking pan with liners.
Prepare the crumb topping
To prepare the crumb topping, stir together the flour, brown sugar, cinnamon, and salt. Add the melted butter. Use two forks to gently mix the butter into the flour mixture, forming pea sized crumbs and some smaller crumbs. Your aim is for there to be no remaining dry flour and for the mixture to be mostly pea-sized crumbs, mixed with some smaller and a few larger crumbs. Pop the crumble topping into the fridge while you prepare the cake batter.
Prepare the muffin batter
In a large mixing bowl, whisk together the flour, salt, baking soda, and baking powder.In another, medium mixing bowl, whisk together the buttermilk, melted butter, cane sugar, and vanilla extract.
Make a well in the center of the dry ingredients. Pour in the wet mixture. Use a spatula to fold the ingredients together, until you have a batter and no visible streaks of flours remain in the bowl. It's OK if the batter has small lumps—don't over-mix! Gently fold the blueberries into the batter with a spatula.
Bake
Use a muffin scoop or 1/2 cup measure to fill the muffin pan (each muffin container should be about 2/3 or 3/4 of the way full). Evenly distribute the crumb topping over the muffins.
Bake for 25 minutes, or until the tops of the muffins are puffy and set and the crumble topping is turning golden. Remove the muffins from the oven and allow them to cool for 10-15 minutes. As soon as the muffins are cool enough to handle, remove them from the pan and transfer them to a cooling rack. Cool for 10-15 more minutes before enjoying.