Make the sponge in a 30 x 40 cm (12 x 16 in) baking tray, lined with baking paper. Leave the sponge to cool completely.
Make the pastry cream and divide equally between two bowls. Sift the cocoa powder into one and whisk in well. Combine the coffee and the liqueur in a separate bowl.
Transfer the sponge from the tray to your work surface. Cut into 8 mm (⅓ in) strips along its wi