Preheat the oven to 180C (Fan oven 160C), Gas Mark 4 and grease and line a shallow, square cake tin. Using an electric hand whisk, beat the eggs and sugar together in a bowl.
Melt the chocolate and butter in a separate glass bowl over a pan of simmering water. Stir the chocolate into the egg and sugar mixture. Add the flour, nuts and marmalade and stir again.
Pour into the prepared tin and bake in the oven for 30 to 35 minutes. Leave to cool in the tin for 10 minutes before turning out onto a wire rack to cool completely.
Cut into squares before serving.