For the crumb topping, stir together the 1 cup All Purpose Flour, 3 tablespoon Brown Sugar,2 tablespoon White Sugar, 1 teaspoon Baking Powder, and 1 teaspoon Cinnamon. Using a pastry blender or your hands, mix in 5 tablespoon Butter (melted) until the streusel is crumbly and well mixed.
For the Cake
Preheat oven to 350°F. Grease an 9x9 pan or a 9-inch springform pan or 9-inch square cake pan. It's better to cover the bottom with parchment paper after greasing for best release.
In a large bowl for an electric or stand mixer, add 1 ½ cup All-Purpose Flour, ½ cup White Sugar, ½ cup Light Brown Sugar, ½ teaspoon Baking Soda,1 teaspoon Baking Powder¼ teaspoon Salt, and ½ teaspoon Ground Cinnamon. Mix at medium-low speed until combined.
Add ½ cup Butter to the flour mixture, 1 tablespoon at a time, not adding the next tablespoon until the previous has been well mixed. Once all the butter has been added, mixture will resemble coarse sandy crumbs.
In a separate bowl, whisk together 2 tablespoon Vegetable Oil, 1 cup Sour Cream, 2 Eggs, and 1 teaspoon Vanilla Extract until mostly smooth. Turn mixer speed to low and slowly add the wet mixture into batter. Stir to combine, scraping the sides and bottom as needed, but don't overmix. Gently fold in 1 cup of Blueberries.
Assembly and Baking
Spread the batter evenly in the pan. Sprinkle the rest of the blueberries over the top and push in just a little.
Sprinkle the remaining crumb topping. You want a mixture of larger chunks and finer crumbs for the best texture of crumb topping.
Bake 50-60 minutes or until the streusel topping is browned and the center is firm. Allow to cool 10 minutes at least and skim a knife around the edge to help release.
After cooling. mix together 2 cups Powdered Sugar and 2 tablespoon Milk in a small bowl for the glaze and drizzle over the cake, if desired.