Line a 21cm square baking tray with baking paper. Lightly grease the paper with a little oil or butter. Put the chocolate chips, condensed milk and peanut butter into a heavy-based saucepan set over a low heat. Stir until melted and thoroughly combined. When the mixture is smooth, add the cashew nuts to the pan and stir them through. Pour the fudge mixture evenly into the prepared tray. Smooth the top of the fudge. Place in the fridge for about 1 hour to speed up the setting process. When the fudge is firm, turn it out onto a board and carefully peel off the paper. Cut into 30 bite-sized pieces and store in an airtight container in the fridge.