Mix the cream cheese, sugar, cornstarch, and salt. Place the cream cheese, sugar, cornstarch, and salt in the bowl of a stand mixer fitted with a paddle attachment. (Alternatively, use an electric handheld mixer and large bowl.) Mix on medium-low speed until the mixture is creamy, like thick frosting, and no lumps of cream cheese remain. Scrape down the beater and the sides of the bowl with a spatula.