Place the chocolate in a saucepan with the butter and golden syrup. Heat gently, stirring occasionally, until the chocolate and butter have completely melted and the mixture is smooth and glossy.
Stir in the cornflakes and mix until completely coated in the chocolate. Spoon the cornflake mixture into 20 paper cake cases arranged on a tray or baking sheet and chill for 2–3 hours until firm. Decorate with sliced mini marshmallows.