To make the ice cream, whisk the egg whites in a bowl with an electric whisk until stiff. In another bowl, whisk the yolks, icing sugar and vanilla seeds until pale and thick like fresh custard. In a third bowl, whip the double cream until just holding its shape. Fold the yolk mixture into the cream and stir in about half of the cherries from the jar, leaving the kirsch behind. Now fold in the egg whites. Pour into a 900g loaf tin lined with a sheet of baking parchment that overlaps each side – you will use this to roll your ice cream later. Freeze for 2 hrs.
Remove the ice cream from the freezer and tin when it feels set but soft. Lift out of the tin on the parchment. Use a spatula to shape the mixture into a rough sausage shape, then use the parchment to roll the mixture tightly together into a cylinder about 35cm in length. Twist the ends of the baking parchment like a cracker and return to the freezer for 1 hr.
Heat oven to 200C/180C fan/gas 6. Line a 24 x 32cm Swiss roll tin with baking parchment (with plenty of excess) and butter well. Whisk the eggs and sugar with an electric hand whisk for 2-3 mins or until thickened and pale. Sift the flour, cocoa powder and baking powder into the egg mixture and add the vanilla extract and fold everything together with a large metal spoon until there are no pockets of flour visible. Scoop 50g of the mixture into a small bowl and add a small amount of red food colouring. Mix well, then transfer to a piping bag. Pipe polka dots onto your prepared Swiss roll tin and bake in the oven for 2 mins. Remove from the oven, then pour the rest of the sponge mixture over to cover the dots and fill the tray completely. Bake for 15 mins or until lightly golden and springy to the touch.
Remove from the oven, allow to cool until cool enough to handle, then carefully roll up the sponge while it’s still warm (keeping the baking parchment attached). Leave to cool completely in its rolled-up shape. Uncurl the sponge and spread the jam on the inside. Working quickly, unwrap the ice cream and put it on top of the jam. Roll the cake up around it.
Wrap the roll in a new sheet of baking parchment and a layer of foil. Return to the freezer for at least a further 20-30 mins, then unwrap, slice and serve.