(click on the text to highlight each stage):

1. Whisk the eggs and sugar until combined. Melt the chocolate and butter in a bowl over a pan of simmering (not boiling) water or in a microwave oven. Stir in the yoghurt. Gently stir into the egg mixture.
2. Sift the dry ingredients together and gently fold into the chocolate mixture until well combined. Pour the mixture into a 23cm/9 inch greased square tin and bake for about 45 minutes Gas mark 4/180ºC/350ºF until cooked through.
3. Leave in the tin for 15 minutes, and then turn out onto a wire rack to become cold. Store in an airtight tin.