Add the heavy cream, dulce de leche, and cornstarch (if using) into the bowl of a stand mixer fitted with the whisk attachment (or a large bowl with a hand mixer).
Beat the ingredients together on medium-high to high speed until they are well combined and very thick, about 3-5 minutes.
The mixture should form soft peaks - if it does not, continue to mix or add an additional teaspoon of cornstarch.
Transfer the thickened mousse to a pastry bag and pipe the mixture into dessert cups, or spoon it in gently if preferred.
Top with an additional drizzle of dulce de leche, if desired, or leave plain.
Can be served immediately, or made ahead and chilled for up to 1 day ahead of time.