Place the frozen raspberries on a plate and allow them to defrost slightly for 10-15 minutes. Preheat the oven to 375°F (190°C) or 340°F (170°C) for fan-assisted ovens. Line two large baking sheets with parchment paper.

In a large mixing bowl, combine the softened unsalted butter, caster sugar, and light brown sugar. Using an electric mixer, beat the mixture on medium speed until it becomes light and creamy.

Add the zest of one lemon and the egg to the butter mixture. Continue beating until the ingredients are fully combined, creating a smooth texture.

In a separate bowl, sift together the all-purpose flour, baking soda, and sea salt. Gradually add the dry mixture to the wet ingredients, stirring with a wooden spoon or spatula until just combined. Be careful not to overmix, as this can affect the texture of the cookies.

Gently fold the finely chopped white chocolate into the dough. Then, carefully incorporate the slightly defrosted raspberries, allowing them to break slightly and marble the dough with streaks of red.

Divide the dough in half. Scoop out portions approximately the size of a golf ball from each half. Arrange the portions on the prepared baking sheets, leaving ample space between each for spreading. Lightly press down on each dough ball to slightly flatten.

Place the baking sheets in the preheated oven and bake for 15-17 minutes, or until the edges are golden brown while the centers remain slightly soft.

Remove the baking sheets from the oven and allow the cookies to cool for 5 minutes. Transfer the cookies to a wire rack to cool completely before serving. Repeat the process with any remaining dough.