To make the base, butter and line a 23cm loose-bottomed tin with baking parchment. Put the digestive biscuits in a plastic food bag and crush to crumbs using a rolling pin. Transfer the crumbs to a bowl, then pour over the melted butter. Mix thoroughly until the crumbs are completely coated. Tip them into the prepared tin and press firmly down into the base to create an even layer. Chill in the fridge for 1 hr to set firmly.

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