Combine ginger, onion, and garlic, and pound to a coarse paste with a mortar and pestle. Set aside.

Melt the butter over low heat, stirring constantly, taking care that it does not darken. Skim off the foam as it rises and continue cooking until all the foam is gone.

Add the paste and all remaining ingredients to the butter and simmer, uncovered, at the lowest possible heat, stirring occasionally, for 30 minutes.

Remove from the heat and allow to cool. Pour off the transparent top layer and reserve it, leaving behind as much of the milk solids and spices as possible. Strain the transparent layer through several layers of cheesecloth. The butter will usually be an oil at room temperature and will solidify in the refrigerator, where it can be kept, covered, for 3 or 4 months.

A World of Curries by Dave DeWitt and Arthur Pais Isbn 0-316-18224-9 pg 182