Begin by peeling and dicing the green mango into small pieces. It's crucial to use an unripe mango to achieve the tartness needed to replicate tamarind's flavor. If your mango isn't quite sour enough, don't worry—you can adjust the tartness later with more lime juice.
In a blender or food processor, combine the diced mango, molasses, lime juice, soy sauce (if using), salt, and 2 tablespoons of water. Blend until smooth, aiming for a thick puree. If the mixture is too thick for your blender, gradually add more water, one tablespoon at a time, until the blender runs smoothly. Be cautious not to add too much water; you want the paste to maintain a thick consistency similar to tamarind paste.
Pour the mango puree into a small saucepan and bring it to a simmer over medium-low heat. Stir continuously to prevent sticking and ensure even cooking.
Allow the mixture to cook for about 10-15 minutes, letting it thicken and the flavors meld together. The consistency should be rich and thick, closely resembling tamarind paste. Keep the heat low and stir frequently—the molasses can cause the mixture to burn if left unattended.
Once the mixture is cooked, taste it to check the balance of flavors. If it needs more tang, add a little extra lime juice. If it's too sour, balance it out with a small pinch of sugar or additional molasses. For a more authentic tamarind aroma, consider adding a pinch of asafoetida during the last minute of cooking.
Let the mixture cool to room temperature before transferring it to an airtight container. Store it in the refrigerator where it will keep for up to 2 weeks. For longer storage, freeze the paste in ice cube trays. This allows for easy portioning whenever you need it.