Chocolate castella (jiggly cake)

Set a larger pan in the middle of the oven and fill it partway with water, not so much that the cake pan displaces it over the edge.

Heat the oven to 350F, 180C, convection fan on.

Cut a square of parchment, snip a diagonal into each corner about as long as the pan is deep, and press it into the 8-inch pan.

Weigh the oil, chocolate chips and milk into a microwave-safe bowl and microwave about 45 seconds until bubbling; whisk smooth.

Add the Dutch cocoa, cake flour and a big splash of vanilla.

Separate the eggs, whites into a large bowl; mix the yolks into the chocolate mixture while still warm so the chocolate does not set into grains.

Add a tiny pinch of salt and a little cream of tartar to the whites and beat until very foamy and leaving trails.

Add the sugar gradually, in doses, beating to a fluffy, stiff meringue with stiff peaks.

Beat a little of the meringue vigorously into the chocolate mixture to lighten it, then fold that into the rest of the meringue in as few strokes as possible until smooth.

Pour into the lined pan to the brim and top with a few chocolate chips.

Set the pan gently into the water bath and bake about an hour total, until a skewer comes out not quite clean.

Lift the cake out by the parchment sling, peel off the paper and cut into rectangles; eat warm.