Preheat the oven to 180°C / 350°F / Gas mark 4.
Lightly butter and line the bases of 2 x 18cm sandwich tins with baking paper.
Whisk the eggs and sugar until light and fluffy.
The mixture should have increased greatly in volume and be thick.
Sift the flour, cornflour, cocoa and baking powder over the surface and fold in gently using a large metal spoon.
Divide the mixture evenly between the tins, spreading the mixture to the edges, then bake for 15-20 minutes until the cakes are well risen and spring back when gently pressed.
Leave to cool in the tins for 5 minutes, then turn out onto a rack.
Microwave the chocolate and coffee together on medium until the chocolate has melted.
Gently stir until smooth, then cool.
Beat the soft cheese with the yoghurt and icing sugar until smooth, then spoon half the mixture into a small bowl stir in the melted chocolate and set aside.
Place one cake on a serving plate and spread with the yoghurt mixture.
Cover with the other layer, then spread the chocolate yoghurt mixture over the top.
To finish, microwave the 15g chocolate on medium until melted.
Stir until smooth, then drizzle all over the cake.