Puree until completely smooth. Transfer to a bowl and set aside.
Wash and clean the blender or food processor and add the plums. Puree them until smooth. If you need, add a little bit of water to help puree them.
Fill popsicle molds with mango cut into small cubes (it is about 1 tbsp in each mold)
Pour the mango mixture into the molds and leave 1 inch from the top for the plum mixture. Then, add the plum mixture.
Place the molds in the freezer for about 4 hours. To remove the popsicles, run the mold under warm water for 10 seconds and then pull the popsicles out of the molds.
Notes
If you do not have fresh mango or plum, you can use frozen fruit. You can blend the fruit from frozen or let them thaw first.
If you do not have a popsicle mold, you can freeze them in small cups and cover it with plastic wrap. Then cut a hole to fit a popsicle stick and insert it when the mixture is semi frozen.
If you have trouble getting the popsicles out of their molds, just keep running it under water and it'll loosen!
To store: Keep the popsicles in the freezer until ready to consume. The popsicles will keep for up to 3 months.