Preheat the oven to 170 degrees Celsius (325 degrees Fahrenheit) Gas 3.

Cream the butter and sugar together until light and fluffy. Whisk in the eggs, one at a time. If the mixture begins to curdle, add a tablespoon or two of the flour. Once all the eggs have been added, whisk in the cool melted chocolate. Fold in the remaining flour and salt, followed by the nuts (if using) and blueberries, until the batter is evenly studded.

Pour the batter into the prepared pan and bake in the preheated oven for 45–50 minutes, or until an inserted skewer comes out just clean. Leave to cool in the pan on top of a wire rack for 10 minutes, before turning out and cutting into 16 squares. Blondies are delicious eaten warm or cold.