Preheat your oven to 180℃ (or 160℃ for fan ovens) and line an 8x8 tin with baking paper.
In a large saucepan, melt the butter, sugar and golden syrup together. Keep the heat low and stir gently until the butter has melted and the sugar has dissolved.
Remove the pan from the heat and add the oats and lemon zest. Mix well.
Press the mixture firmly into your tin and bake for around 20 minutes. The flapjack should be golden at the edges and wobbly in the middle. It'll firm up as it cools so don't worry.
Let it cool in the tin for around 5 minutes then use a knife to gently score lines where you plan to cut it (this will help to prevent crumbling later). Leave to cool completely, around 2-3 hours.
While the flapjack is cooling, prepare the icing by mixing the icing sugar and lemon juice together. If you want it thicker, add more icing sugar or if you want it thinner, add more lemon juice or water.
Remove the cooled flapjack from the tin and drizzle the icing on top. Sprinkle over some extra lemon zest if desired.
Allow the icing to harden then cut the flapjacks into squares. Enjoy!