For the choux, preheat the oven to 210°C/190°C fan. Make the choux according to the master choux method on page 136, adding the baking powder in with the flour if using plain flour.

Line the two tins with baking paper at the bottom only. Spread 200–225g of choux paste in each tin, leaving the top slightly rough and wavy and using a palette knife to help. If you only have one tin, bake one at a time.

Bake for 35–40 minutes until well peaked, golden and crisp. Leave to cool completely in the tins on a cooling rack, then remove.

For the plums, cut the plums into six. In a small saucepan, combine the plums with 50g of the water, the sugar and vanilla. Heat over a medium heat, occasionally stirring so the plums are well coated. Bring the mixture to a gentle simmer, reduce the heat to low and cover the saucepan with a lid. Steam for 4–5 minutes. If the plums are still firm, cook for a further 2 minutes. Mix the cornflour with the remaining water to create a slurry, then stir into the hot liquid. Bring to a bubble so it thickens. Pour into a clean container and leave to cool.

For the mascarpone custard, heat the milk until simmering. Meanwhile, whisk together the whole egg, egg yolk, sugar, cornflour and vanilla. Pour the hot milk over the egg mixture whilst whisking constantly to temper, then return the custard to the stovetop. Cook for 3–4 minutes over a medium heat until boiling, whisking the whole time. Pour into a clean container, then set aside to cool and gelatinise – make sure you put clingfilm or baking paper on the surface so it doesn't form a skin. You want it to be totally cold and firm before continuing with this recipe.

Beat the mascarpone until smooth – it does have a tendency to be a bit lumpy, so you just have to be prepared to work it. Once that’s ready, set aside and beat the custard until smooth and no longer jelly-like. The easiest way to do this is in a stand mixer bowl if you have one.