Defrost the squares at room temperature (it should take approximately 10 minutes). Preheat the oven to 220°C / fan 200°C / gas 7.
Using a small sharp knife lightly score a border around the pastry square 1cm from the edge. Combine the chicken and the tomatoes with the pesto and spread over the pastry, being careful not to go over the border so it will puff up.
Whisk the egg yolk with 1 tbsp of cold water and then brush the border with the egg mixture. Place on a baking tray and cook in the oven for 16-18 minutes or until the pastry is golden brown. Garnish with fresh basil leaves and serve.