Preheat the oven to 180°C, fan 160°C, gas 4. Grease and line an 18cm square cake tin with baking paper.
Put the chocolate in a bowl set over a pan of simmering water for about 5 minutes, until melted. Remove and leave it to cool slightly.
Put the butter and sugar in a bowl and beat with a hand-held electric whisk until light and pale. Gradually beat in the eggs, then add the flour and the melted chocolate and mix until smooth.
Fold in the cherries and pour the mixture into the tin. Bake for 35-40 minutes. Allow to cool completely in the tin before turning out and cutting into 16 squares.