Preparation

• Slowly melt the butter on low heat to ensure none evaporates. Combine the sugars and lemon zest and juice in a mixing bowl, add the melted butter and vanilla extract and stir until relatively smooth.
• Crack in the egg and whisk it all together until emulsified and glossy.
• Sift in the flour, and add the salt and baking soda. Fold it all together.
• Chop the chocolate squares in half and add them all to the cookie dough along with the blueberries. Stir to ensure they are well mixed then cover with cling film and leave to rest in the fridge for 24 hours.
• The next day, preheat the oven to 200°C/180°C fan/395°F. Line a baking tray with parchment and scoop out golf-ball sized chunks of dough, chocolate and blueberries. Bake for 15 minutes.
• Remove from the oven when the edges are deep gold and the blueberries have burst.