• Preheat your oven to 180°C (355°F). Line an 8-inch (20 cm) square tin with parchment paper, with some paper overhanging.

Make the crumb topping:

• Add all the crumb ingredients to a small mixing bowl and mix until combined. Set aside in the fridge until needed. (note 4)

Prepare the blueberries:

• Add the blueberries, flour and sugar to a medium bowl and toss until combined. Set aside at room temperature. If using frozen blueberries, store the mixture in the freezer.

Make the lemon vanilla cake:

• Optional: Add the sugar and lemon zest to a large mixing bowl and rub with your fingertips until fragrant.
• Add the rest of the dry ingredients (flour, baking powder and salt) to the bowl and whisk until well combined. Add all the wet ingredients (yogurt, milk, oil, lemon juice and vanilla) and mix until just combined. Initially, the batter will be very thick, but it will loosen up after a few minutes.

Assemble and bake the cake:

• Pour the cake batter into your prepared pan and smooth the top. Scatter the blueberries on top of the batter (don't press them down). If there are any excess floury bits from the blueberries, sprinkle it on top. Then, sprinkle the crumble on top.
• Bake the cake for 65-70 (if you used fresh blueberries), or 70-75 minutes (if you used frozen blueberries). The cake is ready when the top is golden brown and the blueberries are bubbling. If you insert a toothpick in the center of the cake, it should have no wet batter on it. Or if you gently press the middle of the cake, it should bounce back and feel slightly firm.
• Allow the cake to cool in the pan for 30 minutes then carefully transfer to a wire rack to cool completely.
• Use a serrated knife to cut the cake into slices. Serve the cake as is, or with a scoop of ice cream. Store the cake in an airtight container at room temperature for 1-2 days, in the fridge for up to 3 days or in the freezer for up to 1 month. The crumble will soften over time.