Start the jalapeño sauce first by adding the coriander and cumin seeds to a small sauce pan. Heat on the stove on medium-high heat until the seeds are lightly toasted. Allow the seeds to cool, put them in a mortar and pestle, and grind to a powder.
Cut the chiles in half, length-wise, remove the stems and seeds and roughly chop. For more heat, remove fewer seeds or use spicier chiles such as serrano.
Add all of the jalapeño sauce ingredients to the food processor and blend until the blended components resemble a sauce. Put aside until the sauce is needed a few steps later. Or, make ahead and store in a container with a lid in a refrigerator for up to 3 days.
Thoroughly wash the squash. Cut in half, scrape out the seeds and remove the stem. (Because the skin can be difficult to remove, you need not peel the squash. Once the peel is braised it will be tender enough to eat.) Cut the squash into chunks of equal size so that all the pieces cook evenly. Remember that larger chunks will take longer to cook.
Place a large sauté pan or braiser on the stove burner and adjust the burner to medium-high heat. Add the jalapeño sauce to the pan and ¼ of the coconut milk. Blend the jalapeño sauce into the coconut milk and slowly add and mix in the remaining coconut milk. Add the lime juice and kaffir lime leaf to the sauce and mix to incorporate.
Place the squash pieces into the sauté pan with the peel side down. Make sure that the pieces are mostly submerged in the sauce.
Bring the mixture to a low boil and turn down to very low heat. Place the lid on the sauté pan and simmer until the squash is cooked through (approximately 35 minutes, depending on the squash variety and size of the pieces.) Add more salt or lime juice, as desired.
Serve atop white rice and garnish with thinly sliced fresh chiles.