In a small bowl, mix the espresso powder with hot water until dissolved. Let cool until warm, not hot.
In a large mixing bowl, beat the butter and cream cheese together until completely smooth and lump-free.
Add the brown sugar and granulated sugar. Beat for 3–4 minutes until light and fluffy.
Mix in the egg, egg yolk, and vanilla extract until smooth.
Add the cooled espresso mixture and mix until fully incorporated.
In a separate bowl, whisk together the flour, baking soda, cornstarch, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Fold in the white chocolate chips.
Cover and chill the dough for at least 1 hour, preferably 90 minutes.
Preheat oven to 350°F and line a baking sheet with parchment paper.
Scoop 2–3 tablespoons of dough and shape into tall mounds. Place spaced apart on the baking sheet.
Bake for 11–13 minutes, until edges are set and centers look slightly underbaked.
Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.