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

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

Heat milk: Heat the milk with vanilla and half the sugar in a medium saucepan over medium heat until hot.

Whisk egg: Whisk yolks with remaining sugar, then whisk in the cornflour.

Milk into egg: Slowly pour about 1/2 cup hot milk into the eggs while whisking. Once incorporated, pour in remaining milk and whisk to combine.

Thicken: Pour the mixture back into the saucepan. Cook on medium, whisking constantly until you feel it starting to thicken then immediately turn down to low (2 - 3 minutes). Keep whisking then when you see a lazy bubble appear and burst on the surface, whisk for 30 seconds longer.

Enrichen with butter: Take off the stove. Whisk in butter until smooth. Scrape into a bowl then cover with cling wrap touching surface. Leave to cool while making the batter (Note 5 for custard tips)

Batter: Whisk Wet ingredients in a large bowl. Add the Dry ingredients then whisk until smooth. Remove 1/3 cup of the batter (Note 5) then pour the remaining batter into the prepared pan and smooth the surface. (Bake the 1/3 cup batter in a separate greased ramekin for 25 minutes).

Custard topping: Whisk custard until smooth. Dollop half around the edges of the cake surface then spread inwards with offset knife. Smooth the surface, then scatter with half the blueberries. Mix remaining blueberries into remaining custard, then spread across the cake surface, pressing blueberries in to flatten the surface.

Bake 45 minutes: Place the cake in the oven and bake for 35 minutes. Remove, then use 2 small paring knives to burst some of the blueberries on the surface (1 knife to stop blueberry spinning, the other to burst). Limit bursting time to 1 minute, no longer! Then bake for a further 10 minutes.

Cool: Remove cake from the oven. Fully cool in the cake pan (around 3 hours). Then refrigerate for 12 hours, still in the pan.

Serve! Remove cake from the pan then cut into slices. Best served at room temperature rather than fridge cold!