Preheat the oven to gas 4, 180°C, fan 160°C. Beat the butter with the icing sugar until pale and fluffy, then add the vanilla extract and mix well. Sift in the flour and cornflour, then add the cocoa powder and mix until smooth (if the mixture feels a little stiff, add a few drops of water to loosen).
Line a baking sheet with nonstick baking paper, then roll the mixture into 32 walnut-sized balls. Arrange on the tray, leaving a little gap between each biscuit. Then use a lightly floured fork to flatten slightly and create a striped pattern on each biscuit.
Bake in the oven for 12 minutes, or until firm to touch. Remove from the oven and wait 5 minutes, before transferring to a wire rack to cool completely.
While the biscuits are cooling, make the custard filling. In a large bowl, mix the butter and icing sugar, until combined. In a small jug, whisk the custard powder with the milk to make a smooth paste and gradually add to the butter, beating continuously to form a smooth icing.
To assemble the melting moments, pipe or spread a little icing onto half of the biscuits, then sandwich with the other halves and dust with icing sugar.
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