In a small saucepan over low heat, stir together butter, sugar, and milk until combined.

Raise heat to medium-low and continue stirring until sugar is dissolved.

Once sugar is dissolved, adjust heat to bring mixture to a low boil.

Allow mixture to bubble gently for around five minutes or until mixture thickens slightly.

Remove saucepan from heat and stir in flaxseed and salt.

Allow caramel to cool slightly before storing in a glass, food-safe container. Caramel will thicken after cooling.

Store in an air-tight container in the fridge and warm caramel gently in a saucepan if serving warm.