Pulse flax seeds in a coffee grinder until finely ground. Transfer to a blender.
Pour coconut milk into the blender. Add vanilla extract, stevia powder, xanthan gum, coconut oil, maple extract, and black salt; blend until very smooth, about 2 minutes.
Transfer blender container to the freezer to chill mixture briefly, about 15 minutes.
Pour mixture into an ice cream maker; churn according to manufacturer's instructions, about 25 minutes. Transfer to a lidded container and freeze until firm but still malleable, about 30 minutes. Stir in wheat and barley cereal nuggets until well-incorporated.