Preheat oven to 325 F. Put a kettle of water on the stove to boil. Line a muffin tin with paper baking cups.
When the water comes to a boil, fill a cake pan or baking dish about halfway up the sides with boiling water. Place pan or dish on the bottom rack in your oven. This will create a moist environment for baking the cheesecakes.
repare the crust: Whisk crumbs, ground nuts and sugar together in a bowl. Add melted butter and mix with a fork until mixture resembles wet sand. Press 1 tablespoon of crumb mixture into the bottom of each baking cup. Set aside.
Prepare the filling: Place cream cheese in the bowl of an electric stand mixer and beat on medium-high speed until smooth and creamy, about 2-3 minutes. Reduce speed to medium-low and add the egg yolks, one at a time, until well combined. Add condensed milk, sugar and Key lime juice and continue mixing until completely blended. The batter should be thick and velvety.
Fill each baking cup with the cheesecake batter, and place on the middle rack of your oven. Bake for approximately 23-25 minutes, until cheesecakes are set, but still slightly jiggly in the center – like Jell-O. Remove cakes from the oven and set on a wire rack to cool. When cooled to room temperature, chill cheesecakes in the fridge for several hours to firm up.
Serve with a garnish of fresh whipped cream, if desired.