Preheat oven to 180°C (350°F) conventional (see note 1 if using a convection oven with a fan) and grease an 8x8-inch square cake tin (I use my homemade cake release). Line the bottom with parchment paper as well. Set aside.

Combine light brown sugar, all-purpose flour and cinnamon. Mix until well combined (and no large chunks remain).

Add in melted butter and mix until well combined. You should have a crumble like consistency. Place in the fridge while preparing the cake batter.

Sift together flour, cornstarch, baking powder, baking soda and salt. Using a whisk, mix until well combined and set aside.

In a large bowl combine butter, oil, white sugar and lemon zest. Using a hand or stand mixer on a medium speed, cream for two minutes until light and fluffy.

Add in eggs one by one on a low speed, mixing well in between each addition.

Add the lemon juice, vanilla, and buttermilk, and mix on low speed until well combined. The batter may look a little curdled. Remove any batter from the attachments, as the rest of the mixing will be done by hand.

Add the presifted dry ingredients from earlier and fold them in with a spatula until just combined. Avoid overmixing—mix only until there are no more large lumps or visible streaks of flour.

Add in blueberries and fold until just combined. Do not overmix.

Once the batter is ready, remove the streusel from the fridge. It should have firmed up a bit by now. Break it into small pieces (I do this with my hands). It should resemble coarse sand—slightly chunkier than regular sand—with small lumps throughout.

To assemble the cake, pour the batter into the prepared pan and spread it out evenly. Scatter the streusel over the top, making sure not to leave any gaps—you want the entire surface of the batter to be covered.

Gently tap the cake tin on the counter once to release any large air bubbles. Bake for 40–45 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs on it.

Allow the cake to cool in the tin for 20 minutes, then turn it out onto a wire rack. After that, flip it again onto your serving plate so the cake is upright (see video for demonstration). Allow to cool to room temperature before serving.