In a bowl, thoroughly mix the condensed milk with the egg yolk.
Stir in the coconut flakes then add the butter.
Cook the mixture in a medium sized non-stick saucepan over medium heat, stirring constantly.
When the mixture thickens and starts to lift off the bottom of the pan when scraped with a wood spoon (about 7 minutes), remove from the heat.
Mix in the vanilla extract and spread the mixture onto a greased plate.
Let cool to room temperature.
Grease hands with butter and using a ½ tablespoon for measurement, roll each measure into a ball.
Using the thumb and index finger, mold each ball into a slightly elongated shape that will fit into the prunes.
Gently dredge each coconut kiss in the granulated sugar until totally covered. Set aside.
Place the prunes on a cutting board and using a paring knife, open a slit into the center of the prune almost reaching the bottom but not all the way through.
Nestle a coconut kiss into the cavity you have cut into each prune.
Press gently to ensure a good contact seal between the edges of the prune and the coconut kiss.
Place into paper bonbon cups.