For the Crumb Topping: Combine flour, cornstarch, brown sugar, salt and cinnamon in a medium bowl and mix well. Add vanilla extract to melted butter and stir to combine. Add vanilla butter to dry ingredients and mix well with a spatula or spoon until dry flour is visible and large and small clumps are forming. Cover with plastic wrap and chill for 1 hour.
For the Cake: Preheat oven to 325° F. Line a 9” x 13” cake pan with parchment, so that 1”-2” of parchment hangs over the two longest sides of the pan. These will help you remove the cake later.
Combine flour, cornstarch, baking powder, baking soda and salt together in medium bowl and whisk well to combine. Set aside.
In a separate large bowl, using a hand held mixer, beat together eggs, sugar and vanilla extract until pale and fluffy, about 2 minutes on medium speed. Drizzle in canola oil slowly while continuing to beat with hand mixer. Mix until thoroughly combined.
Add ⅓ of the flour mixture to the wet ingredients and beat briefly until just combined. Add ½ of the sour cream and beat until just combined. Repeat with another ⅓ of the flour mixture and the last ½ of the sour cream. Add the last of the flour mixture and beat to combine making sure no dry flour remains. Give the batter a final stir with a silicone spatula to make sure no flour is stuck to the bottom or sides of the bowl.
Transfer batter into prepared cake pan and spread evenly with silicone spatula.
Sprinkle top of batter evenly with frozen blueberries. Do not press them into the batter.
Sprinkle the top of the blueberry topped batter with the chilled crumb topping. Use your hands to break up any really large clumps as you distribute the crumb topping. Make sure you get the crumb topping all the way to the edges of the pan.
Transfer cake to the center of your preheated oven and bake for 60-75 minutes, rotating halfway through. Cake is done when crumb topping has started to brown, a knife inserted into the center comes out clean or it reaches and internal temperature of 200° F in the center when tested with an instant read thermometer.
Remove from the oven and cool in the pan on a cooling rack for at least 1 hour before removing from pan. Cool completely on cooling rack before cutting and serving.

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