In a small pot, mix together all ingredients except for the frozen berries and mix well with a whisk until the potato starch and agar agar powder are dissolved
Put the pot on medium heat, and heat until simmering (about 5 minutes) while constantly stirring with a silicone spatula from the bottom of the pot to make sure it doesn't burn.
Pour the mixture into four dessert glasses and set aside. Once it is no longer hot to the touch, let it chill and firm in the fridge for 40 minutes to an hour
Meanwhile, place the frozen berries in a clean pot and cook it on medium heat for about 15 minutes to make the sauce. Cook it down to desired thickness, it works well either jammy or more runny like a compote. Let it cool at room temperature
When ready to serve, top each panna cotta with generous spoonfuls of the mixed berry sauce