Cream butter, brown sugar, and granulated sugar together until combined and creamy, then beat in the egg and vanilla extract.

In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt, then slowly mix into the wet ingredients. Fold in the white chocolate chips and dried cranberries, then chill the dough for at least 1 hour to prevent over-spreading.

Remove the dough from the refrigerator; if chilled longer than 3-4 hours, let it sit at room temperature for about 30 minutes to soften.

Preheat your oven to 350°F (177°C) and line large baking sheets with parchment paper or silicone baking mats.

Scoop and roll approximately 1.5 Tablespoons of dough for each cookie into balls, arranging them 3 inches apart on the prepared baking sheets.

Bake for 11-12 minutes until the edges are lightly browned but the centers still look soft; lightly bang the baking sheets on the counter if cookies didn't spread enough.

Allow cookies to cool on the baking sheet for 5 minutes, pressing extra white chocolate chips and dried cranberries into the warm tops for garnish, then transfer to a wire rack to cool completely.

Store cooled cookies in a covered container at room temperature for up to one week to maintain freshness.