For the sponge, preheat the oven to 190`c and line a 23 x 33cm Swiss roll tin with baking parchment. Place the eggs, sugar, vanilla, red food colouring and lemon zest in a bowl and whisk until very light, fluffy and thickened. Sift the flour over the mixture and fold in carefully. Pour the mixture into the lined tin and smooth it out with a spatula until evenly spread. Bake the sponge for 10-12 minutes, or until just firm to the touch. Place a sheet of baking parchment that is slightly bigger than the tray onto the work surface and dust with some caster sugar. Turn the sponge out onto the parchment paper, then peel off the parchment on the bottom of the sponge. Set aside until completely cooled. When ready to fill, spread over the fruit jam, and quickly fill with the mascarpone ice-cream. Roll up tightly and place in the freezer for 30 minutes to set. To serve, dust the top of the roulade with icing sugar and decorate with berries and mint leaves before serving.