Add condensed milk, key lime juice, egg yolks, and vanilla to a glass bowl set over a pot of lightly simmering water. Whisk until thickened, about 6 minutes.
Remove from heat and place over an ice bath to cool. Transfer to a measuring cup and place in the refrigerator to completely chill (should make about 1.5 cups).
Combine the graham crackers, coconut chips, melted butter, and sugar in a separate bowl.
In 4 (8-oz) mason jars, add a spoonful of graham cracker mixture. Pack the cracker crumbs down with a spoon. Top with a layer of key lime filling, then a layer of whipped cream. Top with more key lime filling, a small sprinkling of graham crackers, and a nice dollop of whipped cream. Sprinkle the top with a bit more graham crackers and a slice of key lime.
