Juice one big lemon. Drain lemon juice through a coarse strainer, discard the seeds and fruit bits. Place a funnel (or the funnel part of a coffee machine) over a cup and line a coffee filter inside. Pour in lemon juice and let drain. If the coffee filter gets clogged, change to a new filter. Alternatively, you can use several layers of cheesecloth or a fine strainer to drain the lemon juice. Measure 50 milliliters lemon juice in a bowl. Set aside.
Prepare a cup of filtered water and a silicon blush.
Combine caster sugar and 200 milliliters filtered water in a small stainless pot (or ceramic pot). The deeper the pot, the better. (*see footnote 1) Do not use a pot made of aluminum or cast iron. Heat over medium heat until bringing to a boil.
Add lemon juice. Keep cooking until it reaches a boil again. Turn to low heat and let the solution simmer. I used a heat level of 2.5 out of 10 on an electric stove. (*see footnote 2)
Do not stir the syrup from this point forward.
While simmering the sugar. Dip a basting brush in water and brush along the walls of the pot to let the water run down into the syrup. This helps prevent the sugar from crystallizing along the walls of the pot. You might only need to do this at the beginning of the cooking.
Check on the syrup every 10 minutes during simmering. If you notice any crystal-like masses appearing on the walls of the pot (close to the surface of the syrup), brush water over the mass to dissolve it and let it run down into the syrup.
It takes 40 minutes to 1 hour to cook the syrup. Keep close watch after 35 minutes. You should notice the color of the syrup turning darker and more bubbles appearing on the surface in the last 10 minutes of cooking. The pictures below shows the the syrup after 30 minutes and 40 minutes of simmering.
When the syrup color turns amber, measure with a clean thermometer. The temperature should register 230 to 239 degrees F (or 110 to 115 degrees C). It took me 54 minutes to simmer the syrup. (*see footnote 3)
If the temperature of the syrup raised too fast while its color still pale, you can add more water and let simmering a bit longer.
When the syrup is done, remove pot from heat and let cool completely.
Transfer the golden syrup to a clean, airtight jar and store in room temperature. If you sanitize the jar, the golden syrup can be stored up to a year.
The golden syrup will be ready to use after 24 hours. The taste gets much better after 2 to 3 days. The bubbles will disappear and the sourness will reduce significantly, thus producing a fruity aroma and more concentrated flavor.