Preheat oven to 200°C / 400°F (180°C fan-forced). Line a 20cm springform pan with paper (here's how I do it).

Crumb – Put the flour, sugar, cinnamon and salt in a medium bowl and mix together with a fork. Then add the butter and vanilla, and use the fork to mix until all the flour is wet, then stop mixing. We want it lumpy and crumbly!

Zest the lemon first and put it aside for the batter before measuring out juice for the blueberry tossing.

Blueberries - Toss the blueberries with lemon juice to wet the surface. Sprinkle with sugar and flour, toss to coat. Set aside.

Whisk dry - Whisk the flour, baking powder and salt in a small bowl.

Whisk wet - In a larger bowl, whisk the sugar, vanilla, zest and eggs until the surface is a bit foamy (~ 15 sec by hand). Add the butter and sour cream, whisk until smooth.

Combine wet and dry - Switch to a rubber spatula. Add the flour in 3 batches, folding in between until the flour is mostly incorporated. Then add the milk and stir until combined. Some small lumps is ok!

Assembling – Pour the batter into the pan. Scatter blueberries on top. Sprinkle any leftover flour at the bottom of the blueberry bowl on top of the blueberries. Cover with chunks of crumb, aiming for ~85% coverage. If necessary, enclose powdery bits in your fist to press them into clumps!

Bake for 65 minutes, rotating the pan halfway, until a skewer inserted into the centre comes out clean (blueberry smear is ok!).

Cool 10 minutes in the pan. Remove the cake from the springform pan then cool for at least another 10 minutes before cutting to serve.

Serving - For the best blueberry crumble-cake experience, serve warm with a scoop of vanilla ice cream! Otherwise, be normal and serve it at room temperature. Ice cream or cream also welcome here.