Set the oven to 180 C/ 350 F
Mix the flour. baking powder and soda together.
Put the softened butter, sugar and zest into the food processor and whizz until white and creamy. Add the eggs and then lemon juice. ( the mixture will curdle).
Add the dry ingredients and pulse till combined. Pour onto a bench top and roll into a log, wrap and refrigerate till firm. ( the mixture will be quite sticky).
Cut into thin disks and bake at 180 for about 10-15 minutes. They expand quite a lot. Cut into the shape of your ice cream mould.
To make the lemon parfait. This is a simple ice cream it's success depends on the beating of the yolks and sugar syrup.
Beat the yolks in an electric mixer until very light in colour, thick and fluffy.
Put the sugar and lemon juice into a sauce pan and melt on low without colouring until the sugar has dissolved. Pour the hot sugar syrup over the yolks a dribble at a time while beating until all incorporated. Beat this mixture till cool.
Put the mascarpone in a separate bowl and whisk in 1/4 of the egg yolks to lighten . Lightly whisk the rest of the yolks into the mascarpone. Don't over mix
Clean the bowl and beat the cream to soft peak. Fold the cream into the mascarpone mixture . Pour into a loaf mould or individual moulds.
Beat the whites till lightly foamy and white. Gradually start adding the sugar until the meringue is glossy and thick.
First pipe some meringue onto 6 biscuits. Using a blowtorch brown the meringue or pop under a hot grill till coloured.
Put another 6 discs of lemon thins onto a plate. Top with the Lemon Mascarpone parfait, then top with the meringue layer.
Serve this with fruit ( quinces or raspberries or strawberries or with lemon caramel sauce if you can handle more sugar) Caramelise the sugar till dark brown, add the lemon juice and cool slightly. Whisk in the butter and cool.