To make the crumb topping, whisk flour, sugar, and salt in a small bowl then add in the chopped pecans. Add the butter and using your fingertips, work the butter into the flour mixture until you get pea-sized crumbs. Set aside in the fridge.
Preheat oven to 350 °F (177 °C) and grease an 8x8x2 square baking pan then line with parchment paper so the edges hang off the sides for easier removal.
In a medium bowl, whisk together flour, cornmeal, baking soda, baking powder, and salt. Set aside.
In the bowl of a stand mixer or using a hand mixer, beat the 3/4 cup of sugar and butter together until light and fluffy, about 3 minutes. Beat in the vanilla and mix until well incorporated. Next, beat in the eggs, one at a time, making sure each egg is incorporated before you add the next egg. Beat the mixture for an additional 3-4 minutes until mixture is pale and fluffy.
With the mixer on low, start adding in the flour mixture and buttermilk in alternating additions, starting with 1/3 of the flour mixture then 1/2 the buttermilk, then the other 1/3 of the flour mixture, then the rest of the buttermilk, and finishing with the flour mixture.
Pour half the batter into the prepared pan. Then in a small bowl, combine the remaining 3 tbsp. of sugar with the cinnamon and sprinkle on top of the batter in the pan.
Cover cinnamon sugar mixture with remaining batter.
Toss blueberries in panko in a small bowl then scatter on top of the batter.
Sprinkle crumb topping on top of the blueberries then bake for 55-65 minutes. Let cool completely before slicing.