In a saucepan over medium heat, cook frozen fruit for 2–5 minutes, stirring occasionally, until softened and juicy.
Stir in lemon (or lime) juice, cane sugar, and alternative brown sugar until fully dissolved.
Mash with a fork, potato masher, or immersion blender for desired texture.
Remove from heat. Stir in chia seeds and optional vanilla.
Let sit for 10–15 minutes to thicken, stirring once or twice. Cool, then store in a jar in the fridge for up to 7 days.
Notes
Frozen Fruit: For raspberries I increased the sugar to 1 tablespoon (14 g) to help balance the tartness of the berries. For blackberries I swapped lemon juice lime juice