Line a 6-hole muffin tin with paper cases. Stir the crushed biscuits into the melted butter and press into the bases of the cases. Chill while you make the filling.
Place the espresso powder in a heatproof bowl and pour over the measurement water to dissolve. Leave to cool.
Place the sugar, vanilla extract, cream cheese, cornflour and eggs in a bowl and beat together until smooth, then stir in the espresso.
Spoon the mixture over the biscuit bases and place in a preheated oven, 160ºC (325ºF), Gas Mark 3, for 15 minutes. Leave to cool in the tin for 5 minutes.
Remove the cheesecakes from the cases and place on 6 serving plates. Lightly whip the cream in a bowl with a hand-held electric whisk until it forms soft peaks, then spoon on top of the warm cheesecakes. Dust with the cocoa powder and serve.