Add the dairy free spread (150g) and sugar (90g) to a mixing bowl and beat together with a stand or hand mixer until pale and fluffy.
Mix in the vanilla extract (1 tsp) and salt (pinch).
Sieve in the flour (225g) and stir everything with a wooden spoon or spatula (not the mixer) until it comes together as a dough.
Turn out onto a floured surface and roll out to around 1/2 cm thick. Start cutting out the biscuits with a 2 inch fluted round cutter. Cut out as many as you can, then transfer the biscuits to the tray using a knife or small palette knife. Re-roll the dough and continue cutting out the biscuits until you can't make any more - you want an even number so they can all be sandwiched.
Now cut out a heart from the centre of half of the biscuits. If you don't have the perfect sized heart-shaped cutter (I don't), you can easily make one yourself using tin foil: Roll out a few inches of foil and then tightly fold it up into a long strip (~1cm wide), making sure to really firmly press down the folds. Fold the strip in half in the middle, this will be the indent of the heart shape. Then take a pen or pencil and carefully use it to bend each side of the foil strip next to the fold into a curved heart shape. Trim the excess length at the bottom of the heart and stick the two edges together with tape. Your cutter is now ready to use.
Meanwhile prepare the jam. Sometimes it can be a bit lumpy so spoon out the jam (3 tbsp) into a small bowl and stir well to smooth it out. If it's still lumpy, you can gently heat it up in a saucepan to thin it slightly and melt any big lumps.
Sugar-Free Option: The method is almost exactly the same except for beating the butter on its own first before adding the agave syrup, vanilla and salt. Then sieve your flour in the same way and stir until the dough forms. They also bake for 10 minutes. For a refined-sugar free jam alternative use a fruit spread - these are jams made only from natural sugars. I really love the Meridian Organic Fruit Spreads which come in many options and lack nothing in sweet fruity flavour.
Another alternative is to customise your fillings, you could opt for vegan chocolate spread instead of jam or I really like this lemon curd recipe from The Vegan Society which adds a tangy twist (see last photo).