In a mixing bowl, combine the brown sugar, salt, cinnamon, and flour. Add the butter and with your hands, combine everything until it is well incorporated. You want it to have a shaggy look. This can be stored in an air tight container for up to 7 days in the fridge.
Blueberry Cake:
Preheat the oven to 325° F. Prep a 8 inch round cake pan by spraying the bottom with cooking spray, line with parchment paper and spray again. Set aside.
Make the cinnamon sugar filling by combining the brown sugar and cinnamon in a small bowl. Set aside.
In a medium bowl whisk together the flour, baking powder, and salt. Set aside.
In either a large bowl or a stand size mixer fitted with the paddle attachment, cream sugar and butter together on high until smooth and creamy, about 3 minutes.
On medium speed, begin to add the eggs, one at a time, mixing for about 30 seconds after addition. With a rubber spatula, scrape the bottom and sides after each addition. Add the vanilla and mix on low just until combined.
Add half of the flour mixture and half of the buttermilk mix on low just until combined. Repeat this step. Do not over mix. Just mix until there are little to no flour streaks.
Coat the blueberries with flour and with a rubber spatula gently fold them into the batter making sure not to pop the blueberries as you’re mixing.
With an offset spatula, spread half of the batter onto the prepped baking pan, generously sprinkle the cinnamon brown sugar mixture evenly across the top of the batter. Top with the rest of the batter and sprinkle the streusel on top. Bake for 50-55 minutes. The cake will be done once a toothpick is inserted and little to no crumbs show.