Add the frozen raspberries and maple syrup into a small saucepan over medium-high heat. Bring the fruit to a simmer.
Let it simmer for 5 to 7 minutes, until the fruit has released a lot of liquid and looks like has broken down. Use a potato masher to gently mash and help it along - it should thicken slightly, but it will still be runny.
Mix in the vanilla and salt. Leave to simmer for a few minutes.
Remove from the heat, then stir in the chia seeds. Leave for ten minutes for the chia seeds to thicken.