Place butter, sugar, syrup and peanut butter into a saucepan and heat until the butter has melted, stirring well to combine.
Crush the biscuits until you have fine crumbs and then add to your melted butter mixture. Using the back of a spoon, press your biscuit mixture firmly down into your tin.
Gently melt your chocolate and pour over the biscuit mixture, spreading out evenly. Roughly chop your salted peanuts and sprinkle over the chocolate. Refrigerate until set.