The key to this is trying not to over-fill them. You never need as much jam as you think! A teaspoon might not look like a lot but as the jam heats up and starts bubbling it will spread and fill the pastry case. If you fill the jam tarts to the top before baking they will spill over - so I recommend filling them around half-full. That being said, a bit of jam-splosion adds character, I think!