Line a 1 kg (2 lb) loaf tin with baking paper and place in the freezer to chill.
Heat half the cream gently in a pan. Add the chocolate and stir until melted. Remove from the heat.
Put half the cornflour, half the sugar and 4 egg yolks in a bowl. Put the remaining cornflour, sugar and yolks in a second bowl. Stir, then slowly whisk the chocolate cream into one bowl. Return to the pan and heat gently, stirring until slightly thickened and smooth. Whisk in half the powdered gelatine, then pour into a jug.
Pour half the chocolate cream into the base of the prepared loaf tin. Freeze uncovered, on a flat surface, for 45 minutes until just set.
Combine the remaining cream, espresso coffee and coffee liqueur in a pan over a low heat. Whisk this into the mixture in the second bowl, transfer to the pan and warm through. Add the rest of the gelatine and whisk together. Remove from the heat and pour into a jug.
Pour half the coffee cream over the set chocolate cream and refreeze for another 30 minutes or until just set. Pour over the remaining chocolate cream and freeze for 20 minutes, until just set, then pour on the final coffee cream layer. Chill for 4 hours. Run a knife around the sides, turn on to a board and slice to serve.