Several Indonesian dishes that I've been thinking about making call for Kecap Manis, a sweat somewhat viscous soy sauce. I've seen several potential substitutes online involving normal soy sauce, but I'm not convinced that they would actually make a decent tasting substitute considering how much of the dishes flavor comes from the Kecap Manis. What other sauces would you consider substituting that might give the dish the same Indonesian flavor?
You could try using a different types of sugar with your soy sauce, perhaps palm sugar. This has that different flavor that your probably looking for. This is also the type of sugar that is commonly used when cooking in Southeast Asian foods, such as Indonesian food.
Also the type of soy sauce you are using in the first place, You might want to try a Thai or Filipino style soy sauce, different flavor components. These are probably closer to the Indonesian taste, than say Japanese soy sauces. I find them sweeter and not as salty.
Just as an aside, are you finding it difficult to find this sauce? Or do you just want to try making your own?
Kecap Manis is not simply sugar & soy sauce, as there are spices, too. (typically galangal (related to ginger) is one of them, which I've only found in the US in larger asian/international markets).
You can also find plenty of recipes for it (often intended for Indonesian or Dutch expats in America):
1c. dark brown sugar, 1c. water, 1c. soy sauce, 7TB molasses, 1tsp fresh ginger, 1/2tsp corriander, 1/2tsp black pepper. (see the link for instructions)
1/2c. soy sauce, 3TB molasses, 3TB brown or palm sugar, 1/4 a red chili pepper, 1 small garlic clove, 1" of ginger, 1 star anise (optional), some water. (see link for instructions)
1c. soy sauce; 1c. brown sugar, palm sugar or molasses; 1/2c. water; 1" ginger or galagal (optional); 1 garlic clove (optional); 1 star anise (optional). (do I have to keep saying 'see link'?)
1 part soy sauce, 1 part palm sugar (or brown sugar or molasses), flavorings (garlic, ginger, dried chiles, star anise, curry leaves). (see link)
1c. palm or brown sugar, 1c. soy sauce, 1/2c. water, 1 to 2 tsp molasses (optional), 1 or 2 slices galangal, 1 star anise. (blah, blah)
1 1/4c. sugar, 1/2c. water, 10oz reduced sodium soy sauce, 1 star anise, 1 clove garlic. (ditto)