Preheat the oven to 350 F (177 C). Lightly grease and line the bottom and two sides of a 9 x 9 inch (23 cm) cake pan with parchment paper.
In a large bowl, whisk together the flour, brown sugar, granulated sugar and ground cinnamon for the streusel topping.
½ cup (60 g) all purpose flour, ¼ cup (50 g) light brown sugar, 2 tablespoon (25 g) granulated sugar, ½ teaspoon ground cinnamon
Work in the cold butter cubes until a sand like texture develops. There will be a few bigger crumbs, but most should be finer, smaller pieces. Place the bowl in the fridge until the coffee cake is ready for topping.
4 tablespoon (57 g) cold unsalted butter, cubed
In a medium bowl whisk the all purpose flour, baking powder, baking soda and salt together until well combined.
2 cups (240 g) all purpose flour, 2 teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
In a large mixing bowl, beat the sugar and lemon zest together until they are the consistency of wet sand. Beat butter and lemon sugar together until light and fluffy. This takes about 2-3 minutes on medium speed. On low, beat in the eggs and vanilla extract until smooth and well combined. 1 cup (200 g) granulated sugar, 1 lemon zested, ½ cup (113 g) unsalted butter, room temperature, 2 large eggs (US), room temperature, 1 teaspoon vanilla extract
Mix in ½ of the dry ingredients into the wet, followed by the sour cream, then the dry. Repeat until all the ingredients are just combined. Fold in the flour coated blueberries.
2 cups (300-325 g) blueberries , 1 cup (240 g) sour cream, room temperature
Evenly spread the blueberry coffee cake batter into the prepared pan. Top with the streusel, making sure to break any extra large pieces.
Bake the coffee cake for 45-55 minutes or until a toothpick inserted into the center of the blueberry coffee cake comes out clean or with a few moist crumbs. Cool the pan in the pan for 15-20 minutes. Gently lift the coffee cake out of the pan using the parchment paper. Transfer to a serving plate or serve right from the pan. Top with powdered sugar or a glaze, slice and enjoy!