In a saucepan, whisk together the pineapple puree, sugar and cornstarch. Heat over medium-low heat, whisking, until the sugar dissolves. Place the egg yolks in a bowl. Whisk in a few tablespoons of the warm pineapple mixture, then reduce the heat and add the eggs to the saucepan, whisking the entire time. Cook over low heat until it thickens, stirring constantly, for about 10 minutes.