Step 1

Preheat the oven to 350°F (175°C). Grease and line an 8x8 inch metal square pan with parchment paper and set aside.

Step 2

In a medium bowl, combine the flour, cornstarch, brown sugar, cinnamon, and salt. Add the cold butter and use your fingers or a fork to work it into the dry ingredients until large crumbs are formed.

Step 3

In a large bowl, add the granulated sugar and lemon zest. Work the zest into the sugar by pinching with your fingertips. In a separate bowl, add the blueberries and a tablespoon of the flour and toss it so all the berries are covered in flour.

Step 4

To the lemon sugar, add the remaining flour, cornstarch, salt, baking soda, and baking powder, and whisk together. Set aside.

Step 5

Add the butter into the dry ingredients and work it in with a whisk, a hand mixer, or a stand mixer, until the mixture looks like moist sand.

Step 6

In a separate bowl, combine the eggs, sour cream, and vanilla extract. Use a whisk to mix them until a thick mixture forms. Pour it into the dry ingredients and start mixing. Slowly add the milk and continue to mix, just until you no longer see any pockets of dry flour.

Step 7

Pour ⅔ of the cake batter into the prepared pan. Add the blueberries to the remaining cake batter and fold them in. Gently add this on top of the cake batter layer.

Step 8

Evenly add the crumb topping on top of the cake batter, and bake in the preheated oven for 60-65 minutes, or until a toothpick inserted into the center comes out clean. Let it cool completely before taking it out of the pan to cut and serve.