In a medium saucepan, whisk together the eggnog and cornstarch until smooth. Take a moment to make sure there are no lumps before heating.

Place the pan over medium heat and cook, stirring constantly, until the mixture thickens and begins to bubble. Let it boil for about 2 minutes to activate the cornstarch.

Remove the pan from heat and stir in the sour cream until fully combined and creamy. Set aside to cool to room temperature.

In a separate mixing bowl, beat the heavy cream and sugar together using a hand mixer or stand mixer until stiff peaks form. The whipped cream should be thick and hold its shape when you lift the beaters.

Once the eggnog mixture has cooled, gently fold in the whipped cream until fully blended. Be careful not to overmix — you want to keep it light and fluffy.

Transfer the dip to a serving bowl and chill for at least 30 minutes before serving.