In a large mixing bowl, cream the softened butter, brown sugar, and granulated sugar together for 2–3 minutes until light, fluffy, and smooth. This step is key for a soft, thick cookie.

Add the egg and mix until fully incorporated, scraping down the sides of the bowl as needed. Stir in the vanilla extract until evenly combined.

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

Gradually add the dry ingredients to the wet mixture, mixing on low speed or by hand just until no dry flour remains. Do not overmix, as this will make the cookies dense.

Gently fold in the white chocolate chips.

Very carefully fold in the chopped strawberries. Mix just until distributed; overmixing at this stage can break down the fruit and add excess moisture to the dough.

Scoop the dough into large balls, about 2½ tablespoons each, and place them on the prepared baking sheets, leaving space between each cookie.

Bake for 12–14 minutes, or until the edges are lightly golden and the centers look just set but still soft. The cookies will continue to firm up as they cool.

Remove from the oven and allow the cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.