In a large bowl, whisk together the flour, cornstarch, baking soda and salt.

Pour the melted butter into a medium-sized bowl. Add in brown and granulated sugar, and whisk vigorously, until no sugar lumps remain. Whisk in the egg, the yolk, and add the vanilla extract.

Add the wet ingredients to the dry ones and mix together with a spatula. Fold in the chopped white chocolate. Then carefully fold in the fresh or frozen raspberries.

Roll the dough into balls, place onto a baking sheet covered with parchment paper, and cover with plastic wrap. Chill in the refrigerator for 30 minutes, then allow the dough to soften at room temperature for about 5 minutes before using it.

Preheat the oven to 325°F (160°C) and line two large cookie sheets with parchment paper.

Place the balls onto the prepared baking sheet, spacing them 2 inches apart. Gently flatten the top of each cookie dough with your hand.

Bake each batch for about 10 to 12 minutes, until the edges are set. Allow the cookies to rest on the pan for 5 minutes, then remove to a wire rack to cool completely. If the cookies haven’t flattened as much as you’d like, bang the pan on the counter once or twice and use a large round cutter to 'scoot' the cookies into a perfectly round shape, if desired.