• 260 g (9¼ oz) dark chocolate (55% cocoa), finely chopped
• 135 ml (4½ fl oz) milk
• 40 g (½ oz) yoghurt
• 4 eggs
• 105 g (3¾ oz) caster (superfine) sugar, for eggs
• 4 egg whites
• 160 g (5¾ oz) caster (superfine) sugar, for egg whites
• 135 ml (4½ fl oz) pouring (whipping) cream (35% fat)
• 55 g (2 oz) unsweetened cocoa powder, sifted

Preheat the oven to 150°C (300°F/Gas 2). Grease a 20 cm (8 inch) spring-form cake tin and line the base and sides with baking paper — the paper should protrude about 2.5 cm (1 inch) above the tin.

Put the chocolate in a large stainless steel bowl and set over a saucepan of simmering water, making sure the base of the bowl does not touch the water — the bowl must be large enough to hold the whole cake mix. Allow the water in the saucepan to boil for 2 minutes, then turn off the heat and stir the chocolate while it slowly melts.

Put the milk and yoghurt in a saucepan over medium–high heat and bring to the boil. Turn off the heat — you should have a curdled milk mixture.

Put the eggs and the sugar for the eggs in the bowl of an electric mixer fitted with a whisk attachment. Whisk the eggs at medium speed for about 10 minutes, or until the mixture is very light and has doubled in volume.

In a very clean bowl, whisk the egg whites to soft peaks, then slowly add the sugar for the whites, whisking until soft peaks form a shiny meringue. Be careful not to overwhisk. Place in the refrigerator.

Whisk the cream until soft peaks form and place in the refrigerator until needed.

You should have ready to fold together the melted chocolate, curdled milk, whipped eggs, meringue, whipped cream and cocoa powder. Pour the curdled milk into the chocolate and use a whisk to mix it in, then add the cocoa and whisk to completely incorporate. Fold in the whipped eggs in three batches, making sure you completely incorporate the first batch before adding more — do not worry too much if you can still see streaks of eggs with the following batches. Lightly fold the meringue into the whipped cream, taking care not to knock out too much air. Fold this into the chocolate mix in three batches, making sure you incorporate the first batch before adding more.

Using a spatula, scoop the cake batter into the prepared tin and tap it twice gently on the bench to even out the mix. Bake for 1 hour 15 minutes to 1 hour 30 minutes. (If you can smell the cake cooking within the first 25 minutes your oven is too hot and you need to drop the temperature.) Do not disturb the cake for the first 45 minutes of cooking, after which time you should rotate it to ensure even cooking. You may need to cover the top of the cake with baking paper and lower the oven temperature if the top of the cake is starting to brown. Test to see if the cake is baked by gently placing your hand on top of it and wobbling it a little, you should feel that the cake has set through. Remove from the oven and allow to cool for about 30 minutes in the tin before removing the sides. When completely cool, slide the cake onto a serving plate, to serve.

It is best to use a sharp fine-bladed knife to cut this cake. Have a jug of very hot water, dip the knife in, and leave for about 10 seconds to warm the blade through. Dry the knife on a tea towel (dish towel) before slicing. Repeat this process after every slice for a perfectly clean cut.