Cream together the unsalted butter and cream cheese in a large bowl until smooth and fluffy, about 2-3 minutes.

Add the powdered sugar and vanilla extract. Mix on medium speed until everything is light and creamy, about 1-2 minutes.

Whisk the all-purpose flour, cornstarch, and salt in a separate bowl. Gradually add this dry mixture to the butter mixture, mixing on low until a soft dough forms, about 1 minute.

Fold in the white chocolate chips gently with a spatula.

Chill the dough in the refrigerator for at least 30 minutes.

Mix the red, green, and white nonpareils with granulated sugar in a shallow bowl.

Scoop the chilled dough into balls, using 1 to 1½ tablespoons for each.

Roll each dough ball firmly into the sprinkle mixture until fully coated.

Bake the cookies at 350°F (175°C) on a lined baking sheet for 10-12 minutes.

Cool the cookies on the baking sheet for a few minutes, then transfer to a wire rack to cool completely.