Preparation of Raspberries and Preheating: Begin by placing the frozen raspberries on a plate to defrost slightly for 10-15 minutes. Concurrently, preheat your oven to 190°C (375°F), with a fan setting at 170°C (340°F), and line two large baking sheets with parchment paper.
Creaming Butter and Sugars: In a mixing bowl, combine the softened unsalted butter with the caster sugar and light brown sugar. Using an electric mixer, beat these ingredients until the mixture is creamy and light. Incorporate the zest of one lemon and the large egg into the mixture, beating until thoroughly combined.
Incorporating Dry Ingredients: Sift the plain flour, bicarbonate of soda, and fine sea salt into the bowl containing the butter mixture. Mix these together using a wooden spoon until just combined, ensuring not to overmix.
Adding Chocolate and Raspberries: Fold the finely chopped white chocolate into the mixture. Following the chocolate, gently fold in the slightly defrosted raspberries, allowing them to break slightly and marble the dough with streaks of red.
Forming and Baking the Cookies: Divide the dough in half. From each half, scoop out portions roughly the size of a golf ball, and arrange these on the prepared baking sheets, ensuring ample space between each as they will spread during baking. Gently press down on each dough ball to slightly flatten.
Baking and Cooling: Place the cookies in the preheated oven and bake for 15-17 minutes, or until the edges are golden brown but the centers remain slightly soft. Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a cooling rack to cool completely. Repeat the process with the remaining dough.