Heat a large non-stick pan over a medium heat. Add the rice pops for 4-5 minutes, stirring occasionally, until lightly toasted, then remove from the heat and set aside.
Heat the butter in a large saucepan over a medium heat and cook until beginning to brown and giving off a nutty smell. Lower the heat, add the marshmallows and cook for a further 2 minutes, stirring constantly, until the marshmallows have melted. Remove from the heat and stir through the toasted Rice Krispies.
Line a baking sheet with greaseproof paper. Lightly butter an 8cm circle cutter, then place on the baking sheet and fill with a large spoonful of the mixture. Press down with the back of a spoon until it takes the shape of the mould. Remove the cutter carefully so you have a Rice Krispie circle that's about 1½cm thick. Repeat until you have 12 circles.
Take the frozen yogurt out of the freezer for 15 minutes to soften slightly. Line three 1-litre containers with clingfilm. Transfer one of the tubs of vanilla frozen yogurt to a large bowl and beat until smooth, then stir through the lemon curd. Transfer this lemon curd frozen yogurt mixture into one of the containers, making sure it is in an even layer. Lightly beat the vanilla frozen yogurt and add it to the second container, then repeat with the strawberry frozen yogurt. Return all three to the freezer for at least 1½ hours, until completely solid.
Place the granulated sugar in a small saucepan with 100ml water over a low heat for 3-4 minutes, until the sugar has dissolved. Turn the heat up until it is boiling rapidly, then add the lemon zest pieces and lower the heat. Cook for 1 minute, then remove from the heat. Place the caster sugar on a small plate. Carefully remove the lemon zest pieces from the syrup, letting any excess syrup drip off. Toss in the caster sugar until completely coated.
Once the frozen yogurt mixture is solid, cut out 2 circles from each flavour using the 8cm cutter. They should be about 2½cm in thickness. For the lemon frozen yogurt disc, roll one side in candied lemon, so that half of it is coated and the other half is still plain. For the vanilla yogurt, roll half the disc in pistachios, and for the strawberry frozen yogurt disc, roll it in freeze-dried raspberries. Return the discs to the freezer until you are ready to serve.
When you're ready to serve, sandwich the frozen yogurt between two of the Rice Krispie circles.
Please note: the nutrition below is for the lemon-flavoured frozen yogurt sandwiches.