Add the butter to a heavy-bottomed pot and set it over a medium heat. Stir occasionally until all the butter has melted.
Turn the heat down to low and let the butter simmer gently. You will start to see a lot of foam rising to the surface at this stage. Just stir it through and let it continue simmering.
About 10-15 minutes in, you will see the milk solids at the bottom of the pot. This will get strained out later. For now, just continue simmering until it turns a deep golden colour. This will take anywhere between 20 - 30 minutes depending on the intensity of the flame and the type of pot used.
Once it starts to darken slightly, keep an eye on it. If there is too much foam on the surface, stir to see the colour underneath. Take it off the heat when it turns a deep golden colour. It will continue to darken slightly as it sits, so use your best judgement here. It will start to take on a different aroma at this stage too.
As soon as you take the ghee off the heat, add the curry leaves, sprig and all. Let it sit uncovered until it comes to room temperature.
Strain through a fine sieve or a sieve lined with muslin. Store in a jar for up to 2 months or in the fridge for up to 4 months.