• Preheat your oven to 180°C (350°F). Line a 23 cm (9-inch) springform pan with baking paper and set it aside.

Crumble Topping

• Melt the butter in a small heatproof bowl in the microwave, then set it aside to cool slightly.
• In a medium bowl, mix together the flour, brown sugar, and cinnamon until combined. Pour in the melted butter and stir with a spatula until a thick dough forms.
• Use your fingers (carefully, if the butter is still warm) to rub the mixture together until it looks even and crumbly. Break up any big pieces to make nice small crumbs, then pop the bowl in the fridge while you make the cake.

Sour Cream Cake

• In a large bowl using a hand mixer (or a stand mixer with the paddle attachment), beat the soft butter, sugar, and lemon zest on medium speed for 3 to 5 minutes, until the mixture becomes pale and fluffy. Don’t forget to scrape down the sides of the bowl as you go.
• Add the eggs one at a time, mixing on medium-low speed after each until fully combined. The mixture should look thick and smooth.
• Next, mix in the sour cream and vanilla extract until just combined.
• Sift in the flour, baking powder, baking soda, and salt. Mix gently until everything is just incorporated. The batter should be thick and airy.
• Assembling the Cake
• Pour the batter into your prepared pan. Spread it out evenly using a spatula or the back of a spoon.
• Scatter all the blueberries evenly on top of the batter.
• Take the crumb topping out of the fridge and break up any large chunks with your fingers. Sprinkle it evenly over the blueberries.
• Bake for 45 to 50 minutes, until the crumb topping turns golden and a toothpick inserted in the center comes out clean.
• Place the pan on a wire rack and let the cake cool completely before removing it. For a pretty finish, dust the top with powdered sugar before serving.