Preheat oven to 180°C/350°F (160°C fan), placing the shelf in the middle of the oven.

Grease a 20 cm / 8" springform pan with butter and line the base and sides with baking paper.

In a saucepan, heat the milk, vanilla bean paste, and 1/3 cup of superfine/caster sugar over medium heat until it’s warm but not boiling.

In a bowl, whisk together the egg yolks and cornflour until smooth. Slowly pour in a bit of the warm milk mixture while whisking to temper the eggs. Then pour the egg mixture back into the saucepan.

Cook the mixture over low heat, stirring continuously until it thickens. Remove from heat, stir in the cold butter cubes until melted, and set aside to cool slightly.

In a large bowl, beat the 1/2 cup of superfine/caster sugar and canola oil together. Add in the large egg and mix well.

Stir in the yogurt, vanilla extract, and lemon zest until combined.

In another bowl, whisk together the flour, baking powder, and a pinch of salt. Gradually add these dry ingredients to the wet mixture until just combined.

Gently fold in the blueberries, being careful not to break them.

Pour half of the cake batter into the prepared springform pan, followed by the custard layer, and then top with the remaining cake batter.

Bake in the preheated oven for about 40-45 minutes, or until golden and a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes, then remove it and let it cool completely on a wire rack.