Start by making the jar: rinse the strawberries with cold water, and pat them dry with kitchen paper.
Remove the stems, then cut the strawberries into quarters and add them to a pan together with the sugar.
Mix well, and leave them to cook on a low to medium heat until the strawberries release their juice.
Use a potato masher to mash the strawberries, and continue to cook them until the mixture thickens lightly, but there is still plenty of liquid left.
Add the chia seeds, and mix well.
Remove the pan from the heat, and allow the jam to cool down completely while the chia seeds bloom and thicken the jam further.
Break the chocolate bars into pieces, and a add them to an electric melting pot to melt into a smooth mixture.
Alternatively, add the chocolate pieces to a heatproof bowl set over a saucepan of gently simmering water, making sure the heat is kept low to prevent the chocolate from becoming grainy.
Once the chocolate has melted and is smooth, add a teaspoon of chocolate to each skull mould, making sure each mould is well coated in chocolate.
Refrigerate until the chocolate sets, about 10 minutes.
Fill each chocolate shell with the jam, and top with more melted chocolate to seal well.
Refrigerate the skulls for at least one hour before unmoulding.