The batter is meant to be thin. Once the boiling water goes in it will look far too runny. That thin batter is exactly what produces the moist crumb. Do not add extra flour.

Make your own custard if you have 15 minutes. Whisk 4 egg yolks with 50g caster sugar and 2 tsp cornflour. Heat 500ml whole milk with 100ml double cream and a split vanilla pod until steaming, pour it slowly onto the yolks whisking constantly, return to the pan and stir over a low heat until it coats the back of a spoon. Never let it boil.

The coffee is not a coffee flavour. One teaspoon in a cake this size simply makes the chocolate taste deeper. Leave it out if you are baking for children.

Traybake version. Bake the whole batter in a lined 20 x 30cm tin for 35 to 40 minutes, then spread the frosting straight over the top.

Serve it warm. Twenty to twenty-five seconds in the microwave per slice makes the frosting molten. With warm custard over the top it becomes a completely different pudding.

Storage. Airtight container at cool room temperature for up to 3 days. Refrigerating dries the cake and hardens the frosting. Unfrosted sponges freeze well for up to 3 months, wrapped in cling film then foil.

Equipment. Two 20cm round cake tins, baking paper, a large mixing bowl, a balloon whisk, a sieve for the icing sugar and cocoa, a wire cooling rack, a bread knife for levelling and a palette knife for the frosting.

Allergens. This recipe contains wheat (gluten), milk, egg and soya (most dark chocolate contains soya lecithin). Some cocoa powders and chocolates carry nut and peanut cross-contamination warnings. Always check the label of every ingredient you use, as recipes and allergen information can change. People with food allergies or intolerances must also consider cross-contamination.

This is a home recipe inspired by the Chocolate Fudge Cake with custard at The Cozy Bowl. It is not the restaurant’s exact commercial recipe.