Cake: Preheat the oven to 160°C fan / 350°F / Gas 4.
Line a 20cm (8inch) springform pan, lined with baking/parchment paper.
Melt the chocolate in a heatproof bowl over a pan of barely simmering water (make sure the bowl isn't touching the water). Stir until melted. Alternatively, melt in the microwave, stirring every 30 secs. Leave the melted chocolate to cool for 5 mins.
Ensure the steam or water is not able to get into the bowl with the chocolate, as this will spoil it.
In a clean mixing bowl add the yoghurt, sugar, oil and eggs.
Gently add the melted chocolate. Whisking to stop the eggs cooking.
Sift the flour and cocoa and add to the bowl. Mix together to form a nice batter.
Pour the batter into your baking pan and place in the oven for 35 to 40 minutes or until a skewer or knife inserted into the centre of the cake comes out clean. Once baked leave to cool.
Buttercream: Beat the butter and icing sugar together with a fork or in the mixer if you have one. Sift in the cocoa and mix again until smooth.
Spread the buttercream over the cooled chocolate cake to get a nice smooth finish evenly across it.
Place in the freezer to set for at least an hour before pouring over the mirror glaze.
Mirror glaze: Soften the gelatin in some cold water.
Place the 50 g water and sugar into a small saucepan and bring to a boil on medium heat.
Add the sifted cocoa powder and whisk.
In another saucepan bring the cream to the boil.
Add the cream into the cocoa sugar syrup.
Add the softened gelatin and whisk until it has dissolved entirely.
Leave to cool until it reaches body temperature at 37°C / 98°F.
To glaze, place the cake on a wire rack over a pan, to collect the excess glaze and pour the glaze on the cake starting from the centre towards the edges with a circular movement.