Mix the cornstarch with one cup (240 ml or 8 oz) of milk in a large bowl until it is completely dissolved.

Add the arequipe or dulce de leche and keep stirring until most of it dissolves. You can also blend it in a blender. Set mixture aside.

In a large pot heat the remaining amount of milk (3½ cups or 840ml) with the white sugar over medium heat until it starts to boil. Once it is boiling remove it from the heat and let it rest for about 5 minutes. Then, put it back on the burner over medium low heat, add the arequipe mixture and start stirring constantly with a wooden spoon or spatula until it thickens and you can see the bottom of the pot. This can take anywhere from 10-15 minutes.

Pour immediately into a heat proof dish or dishes if serving individual portions and let it cool completely before serving.