Preheat oven to 350ºF and crush the graham crackers in a food processor until you've got crumbs. Alternatively, place the graham crackers into a plastic bag and bash with a rolling pin until crumbs form if you don't have a food processor.

Melt the butter in a medium-sized microwave-safe dish and stir in the graham crackers until they've absorbed all the liquid.

Press the buttery cracker crumbs into the bottom of a 7" silicone cake pan and set to one side. This will be your pie base.

In a separate bowl, beat together the condensed milk, egg, lemon juice and lemon zest.

Pour the pie filling on top of the pie base and bake in the pre-heated oven for 15 minutes.

Leave to cool in the cake pan for 30 minutes then chill in the fridge for at least 4 hours before slicing into 4 and serving.