Break the chocolate into a bowl, add the Stork and melt over a pan of hot water (or in a microwave on medium power).
In a large bowl, whisk the eggs with the sugar and vanilla until creamy and slightly thickened.
Sieve the dry ingredients and add to the eggs with the chocolate mixture, yogurt and nuts. Mix gently.
Pour into a greased 22cm (9 inch) square tin.
Bake in a preheated oven at 160°C, 325°F, Gas Mark 3 for 45-55 minutes.
Leave to cool before cutting into squares.