Soak 4 gelatine leaves in cold water for 5 minutes to soften. Meanwhile, place sugar and 1 cup (250ml) water in a saucepan over low heat, stirring to dissolve sugar. Increase heat to medium and simmer for 4 minutes until thickened slightly, then whisk in cocoa.
Squeeze the excess water from the gelatine leaves and add the leaves to the syrup, stirring to dissolve. Add the sherry or marsala and mix well. Strain and set aside to cool slightly.
Place biscuits in a trifle bowl. Pour over cocoa syrup, pressing the biscuits down well. Cover and chill in the the fridge for 2-3 hours to set.
For the second layer, soak the remaining 2 gelatine leaves in cold water for 5 minutes until softened. Meanwhile, chop 3 mars bars and place in a pan with the milk over low heat, stirring until melted and smooth. Squeeze excess water from the gelatine, then add leaves to the milk mixture and stir to dissolve. Cool slightly, then pour over the chilled layer and return to the fridge for 1-2 hours until set.
To serve, spread the whipped cream over the trifle, then top with reamining sliced Mars bar.
Notes & Tips
Gelatine leaves are available from gourmet food shops - check packet for setting instructions Serve in a large glass bowl