Melt 300g dark chocolate in a bowl over a simmering pan of water and let it cool slightly.
Preheat your oven to 170°C.
Rub butter into the flours, salt, baking powder, and sugar before stirring in milk to form a dough ball.
Roll the dough to 5mm thickness and cut six 7.5cm rounds.
Chill the biscuit rounds in the fridge for 10 minutes.
Bake the biscuits for 10 to 12 minutes until they are hard.
Cool the baked biscuits completely on a wire rack.
Coat the inside of the silicone moulds with the melted chocolate using the back of a spoon.
Set the chocolate-lined moulds aside at room temperature to harden.
Coat the cooled biscuits entirely in more melted chocolate and place on parchment paper.
Whisk marshmallow ingredients over simmering water for 8 minutes until stiff and doubled in volume.
Spoon the stiff marshmallow mixture into a piping bag.
Melt the remaining chocolate and place it into a separate piping bag to cool slightly.
Move the chocolate-coated biscuits to a clean sheet of parchment, flat side down.
Pipe the marshmallow into each chocolate-lined mould up to the top rim.
Snip the end off the chocolate piping bag.
Pipe chocolate over the marshmallow and the biscuit rim, then press the biscuit onto the dome.
Allow the teacakes to set at room temperature until completely sealed.
Carefully unmould the set teacakes from the silicone.
Store the teacakes in a cool place, avoiding the refrigerator to maintain shine.