Grease and line an approximately 20 by 30cm (8 x 12 inch) rectangular baking tin, 3 to 4cm (1/2 inches) deep, with greaseproof or parchment paper.
Melt the butter and chocolate either in a bowl over a pan of simmering water or gently in the microwave.
With an electric hand whisk, beat together the eggs, sugar and vanilla extract until they are lovely and thick and creamy. Mix in the melted chocolate and butter. Finally stir in the flour, salt, cherries and walnuts.
Pour into the baking tin and cook for about 25 minutes until the top is cracking and the centre is just set. Leave to cool in the tin for about 20 minutes before cutting into squares.
Serve warm or, if you can resist leaving them, cold.