Preheat the oven to 150C. Grease a 20 cm cake tin (if you have a springform please use that) and line the base and sides with baking paper - the paper should come about 2.5 cm 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 chocolate to sit over simmering water without stirring until almost melted. Remove from heat, mix until smooth.
Put the milk and yoghurt in a saucepan over medium-high heat and bring to the boil.
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 5 minutes, or until the mixture is very light and has doubled in volume. Set aside.
In another very clean and dry bowl of an electric mixer, whisk the egg whites to soft peaks, then slowly add the sugar for the whites, whisking until soft peaks form a shinny meringue. Be careful not to overwhisk. Refrigerate until needed.
Whisk the cream until soft peaks form. Refrigerate until needed.
You should now have separate bowls of 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
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 45 minutes to 1 hour till done.
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, but it didn't come to this for me. 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 the 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.