Adjust oven rack to middle position and heat oven to 350 degrees F
If making the whipped topping: combine cold heavy cream, sugar, and vanilla extract in the bowl of an electric mixer. Using an electric mixer, whip cream for 1 minute., on medium- high speed. Now, increase speed to high and whip until cream is smooth and thick . After 1 minute, check to see if the cream clings to beaters and makes soft peaks that stand up on their own. If not, continue beating and check again in 30 seconds. Transfer whipped cream to air tight container and refrigerate until ready to use.
Place graham cracker in large zipper-lock plastic bag. Press out as much air as possible from bag and seal bag. Use rolling pin to gently crush graham crackers into crumbs . Set aside.In small microwave-safe bowl, combine graham cracker crumbs and butter. Heat in microwave until crumbs are toasted, about 1 minute. Stir with rubber spatula to combine.
Now, whisk together sweetened condensed milk, sour cream, lime juice, and lime zest in a separate medium bowl. Place ramekins on a baking sheet and pour filling into individual ramekins, bake for 10 minutes. Remove baking sheet from oven and place on cooling rack . Let cool for 45 minutes.
Cover cooled ramekins with plastic wrap. Place in refrigerator until filling is chilled and firm, at least 1 hour or up to 2 days. To serve, dollop with whipped cream (if using), and sprinkle with graham cracker topping.
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Notes
You can make the key lime dessert cups up to 2 days ahead of time! After key lime dessert cups have sufficiently cooled, cover with plastic wrap and refrigerate until filling is firm, at least 1 hour or up to 2 days.