Lay the biscuits out on a wire rack and spread a small teaspoon of the jam in the centre of each one.
Place the sugar, egg white, cream of tartar, vanilla extract and 25ml of water into a medium heatproof bowl and beat with an electric handheld whisk for a minute until the mixture turns opaque.
Sit the bowl on top of a pan of simmering water, shallow enough that the water doesn’t touch the bowl.
Using the electric whisk, beat the mixture for 5-6 minutes until it doubles in volume, becomes thick and glossy and can form stiff peaks.
Remove from the heat, spoon into the piping bag and pipe a 4cm-high peaked dome on top of the jam on each biscuit, to come nearly to the edge.
Pop the tray into the fridge for 30 minutes so the mallow firms up a little.
Meanwhile, snap the chocolate into a medium heatproof bowl and either melt in the microwave in 20-second blasts, stirring between each go, or over a pan of simmering water, shallow enough that the water doesn’t touch the bowl.
When melted, remove from the heat and leave to cool to body temperature.
When everything is ready, hold a tea cake over the bowl of melted chocolate and spoon generously with chocolate to coat all over.
Return to the rack and repeat for all.
Chill in the fridge for 30 minutes until set firm.
These will keep for a couple of days in the fridge. Nibble on them at your pleasure!