Put the graham crackers in a mixer and mix for a few seconds until fine crumbs are formed. Alternatively, add the crackers to a freezer bag, seal it and crumble with a rolling pin. In a bowl, mix crumbs, melted butter, salt and sugar. Put 1/3 cup of the crumbs aside for the topping. Press the remaining mixture firmly into the form. Refrigerate for 30 minutes.