First, make the chocolate biscuit bases. Put the butter and icing sugar in a large bowl, mash together with a wooden spoon then beat for 1-2 mins or 2 until smooth. Add the egg yolk, vanilla, milk, flour, cocoa and a pinch of salt, and mix again. Tip onto your work surface and knead briefly until evenly coloured. Wrap in cling film, pat into a disc and chill for 20 mins.
Heat oven to 180C/160C fan/gas 4 and line 2 baking sheets with baking parchment. Dust the surface with a little flour, unwrap the dough and roll to the thickness of a £1 coin. Use a 5cm cookie cutter to stamp out as many discs as you can, then re-roll the trimmings to cut out more – you should get about 35 in total. Place on the baking sheets and bake for 10-12 mins, swapping the trays over halfway through. Cool on a wire rack.
Next, make the marshmallow filling. Put the gelatine in a bowl of cold water and set aside to soften. Put the egg whites, sugar, 1 tbsp water and a good pinch of salt in a heatproof bowl. Place over a pan of gently simmering water, making sure the bowl doesn’t touch the water, and whisk until thick and leaving a prominent trail from the beaters – this will take about 5 mins. When the meringue is thick, remove the bowl from the heat and continue whisking while you add the gelatine leaves, one at a time. Keep whisking for another 3-5 mins until the meringue has cooled slightly and is really stiff. Transfer to a piping bag fitted with a large round nozzle (mine was 1.5cm).
Flip each biscuit over so the bottom is facing upwards. Spoon 1/2 tsp jam onto the middle of each biscuit. Holding the piping bag above the jam, pipe a blob of meringue to fill the surface of the biscuit, quickly pulling away to make a spike. Leave to set for 30 mins.
Melt the chocolate in the microwave, stirring every 20 secs or so to ensure it doesn’t burn. To cover the teacakes in chocolate, hold each one above the bowl at an angle and drizzle over the chocolate, letting it run back into the bowl, turning the teacake until completely covered. Place on a wire rack suspended over a baking tray. After every 5-10 teacakes, sprinkle over a little freeze-dried raspberry, if using. Will keep for 3 days in a sealed container.