This stripy Swiss roll is definitely a worthy party piece. Here, it comes in black and orange, but you could use any contrasting colours to match your theme.
Make the jam. Tip the raspberries and strawberries into a medium pan. Add the sugar and crush the berries and sugar together using a potato masher. Heat over a medium heat, stirring until the sugar has dissolved, then bring to the boil. Boil until the jam reaches setting point (102°C–105°C/216°F–221°F on the sugar thermometer). Carefully pour the jam into a shallow tray and leave to cool, then chill to set.
For the sponge, heat the oven to 200°C/180°C fan/400°F/Gas 6. Crack the eggs into a large bowl with the caster sugar and whisk until the mixture is thick, light and frothy and leaves a ribbon trail when you lift the whisk.
Divide the mixture equally between 2 bowls. Whisk some orange food colouring into one of the bowls until the mixture is bright orange, then whisk some black food colouring into the other bowl until the mixture is jet black.
Sift 50g of the flour over the orange mixture and carefully fold it in using a large metal spoon or rubber spatula. Sift the remaining flour over the black mixture and carefully fold in as before.
Spoon each mixture into a disposable piping bag and secure the end of each bag with a band. Snip the ends so that the opening for each bag is about the size of a 20p piece.
Pipe a straight line of orange cake mixture along the long edge of the prepared tin, then pipe a line of black cake mixture next to the line of orange. Continue piping lines, alternating the colours, so you end up with about 9 (in total) orange and black stripes.
Bake the sponge for 10–12 minutes, or until it is springy to touch and starting to shrink away from the sides of the tin.
Lay two sheets of baking paper on a work surface. As soon as the sponge is cooked, turn it out onto one sheet of paper and carefully peel off the lining paper. Lay the other sheet of paper over the top and flip over the sponge so that the base (that had the lining paper on it) is underneath. Set aside to cool.
Meanwhile, make the filling. Use a hand-held electric whisk to whip the cream to soft peaks.
To assemble, trim the edges of the sponge with a sharp knife and score a line across the sponge 2cm in from one of the short edges. Spread the jam evenly over the sponge, then spread two thirds of the cream in an even layer over the jam. Roll up the Swiss roll firmly, starting at the scored end and using the paper to help you.
Transfer the remaining cream to the large piping bag fitted with the star nozzle. Pipe the cream over the top of the roll and scatter with coloured sprinkles, if you wish. Serve in slices.