Preheat the oven to 180°C (160°C fan oven, 350°F, gas 4). Line 20 cup cake tins with cake papers. Sift the cocoa into a bowl and gradually whisk in 225 ml (8 fl.oz) cold water. Set aside.
In a large mixing bowl, beat the butter with the sugar until well blended.
Whisk in the eggs, then sift the flour and baking powder on top. Using a large metal spoon, carefully fold the flour into the cake batter along with the liquid cocoa mixture.
Spoon into the cake cases to three quarter fill them and bake on the middle shelf in the oven for about 25 minutes until risen just above the top of the cases and just firm to the touch. Transfer to a wire rack to cool.
Put the caster sugar into a heatproof bowl. Add the liquid glucose and the 2 egg whites. Whisk for 1 minute until well blended.
Place the bowl above a saucepan of gently simmering water and whisk for 2-3 minutes until thick. Add 50ml (2fl.oz) warm water and continue to whisk for a further 3-4 minutes until glossy, thick and peaking like a stiff meringue.
Remove from the heat and whisk in the vanilla. Spoon into a piping bag fitted with a 5mm/ 1/4inch plain nozzle and pipe a neat mound of frosting on top to resemble frothy milk or whipped cream on a cup of hot chocolate. Dust lightly with cocoa powder and decorate with mini marshmallows.