4 cups (300 ml) condensed milk
2 cups nangka preserves (finely chopped)
1 cup cornstarch dissolved in
¼ cup water

Add condensed milk to nangka preserved. Cook in low heat. Halfway, from heat, add cornstarch (dissolved in a little water). Blend well, then, continue cooking while stiring constantly. Cook until it thickens and mixture separates from pan. Cool.
Note: When using bottled preserves, before using into balls, set aside cooked mixture for 4-5 hours (or overnight), covered with a cheese cloth. This would present your balls from becoming watery in a short time.