Begin by breaking up the chocolate into pieces, before melting in the microwave, in 30 second bursts. Do not heat any longer than 30 seconds or you run the risk of burning and ruining your chocolate. Mix in-between each 30 second burst. Alternatively, you can melt the chocolate in a bowl on top of a pan with barely simmering water. See Photo 2 below.
Once melted, add in your milk and then the cocoa gradually, whisking together until relatively smooth. See Photo 3 & 4 below.
Pre-heat your oven to: 170°c/150°c Fan Oven/325°c/Gas Mark 3.
Next, mix lightly by hand, the softened cubed butter with the sugar, before beating with your hand/stand mixer till creamed together. The mixture will be spreadable and relatively smooth. Don't worry at this stage, about over-mixing. You want to incorporate as much air into the batter as possible. See Photo 5 below.
Then beat in the egg yolks, yoghurt and melted chocolate mixture, mixing well in-between each addition. Then beat for 1 minute until some volume is created. See Photo 6.
Sieve over the flour and Bicarbonate Of Soda (Baking Soda), and fold in gently by hand. Fold like the spatula is your pen and the batter your paper, as if you were writing a number '8'. This helps to reduce knocking out air you already incorporated into the batter. See Photo 7 below. A rubber or silicone spatula is advised, rather than something metal.
In another bowl, whisk your egg whites, till firm peaks are formed. Be sure to use a bigger bowl than would normally fit the egg whites, as they will increase in volume. See Photo 8.
Finally, fold the whisked egg whites into the prepared cake batter. Do so in increments of 3. Folding in a ⅓ of the whites to begin with, will be a little easier, and loosen the batter a little, making the next 2 additions easier to incorporate. Gradual and gentle folding, again to stop knocking out the air that is in the whisked egg whites. Fold only until no more white is visible and be sure to check the bottom of the bowl.