Peel and slice the purple sweet potatoes into medium pieces and boil until tender. Drain and mash.
In a deep saucepan combine all the ingredients except the butter. Place on medium-high heat and stir non-stop while the mixture thickens, it can take anywhere between 10 and 20 minutes. After, add the butter and mix it in well.
When the mixture starts to pull away from the sides of the pan and a spoonful doesn’t fall, it is ready. The goal is to achieve a paste consistency.
Keep 3/4 cup aside for the sesame seed balls and pour the rest into a well-buttered dish. Refrigerate for 1 hour before serving and spronkle toasted coconut on top.
Sesame Seed Balls
Mix the glutinous rice flour with the sugar, salt, and water. Knead just enough to form a ball. Divide dough into small pieces and form small balls, about 12.
Roll each dough ball into a flat disc then place 1 tablespoon of the filling in the middle. Wrap the dough around the filling and pinch shut. Roll the ball in moistened sesame seeds. Repeat.
In batches, deep fry in hot oil at 320 F for about 10 minutes, or until the balls float. Keep the balls submerged in the oil with a slotted spoon as you slowly increase the oil to 350 F, and fry until golden brown, about 5 minutes. Drain on a paper towel and leave to cool a bit before serving.