Preheat the oven to 350 degrees F (175 degrees C). Lightly butter a baking pan.
Combine butter, sugar, and golden syrup in a saucepan over low heat. Mix until butter has melted and sugar has dissolved. Remove from heat. Add 2 cups oats and raisins. Mix until oats are well coated. Pour mixture into the prepared pan; flatten down with the back of a spoon.
Bake in the preheated oven until golden brown on top, 10 to 20 minutes. Let flapjack cool.
Melt chocolate in a microwave-safe glass or ceramic bowl in 15-second intervals, stirring after each interval, 1 to 3 minutes. Pour over the cooled flapjack. Let cool until chocolate is set.
