Step 1

Place the chiles in a medium, heat-proof bowl and then pour boiling water over them to cover. Let stand for 15 to 20 minutes, until the chiles are very soft.

Step 2

Remove the chiles from the soaking water using tongs or a slotted spoon, reserving the soaking water. Transfer the chiles to a blender.

Step 3

Add the tomatoes, onion, garlic, and salt to the blender with the chiles. Add ½ cup of the soaking liquid (or use water) and process until smooth. Add additional soaking liquid or water as needed to get a saucy consistency. If the mixture is too spicy, add additional tomatoes.

Step 4

Heat the oil in a large skillet over medium-high heat. Add the shrimp and cook, turning over once, until they are pink and opaque, about 3 minutes total.

Step 5

Add the sauce to the pan, reduce the heat to medium, and simmer for about 5 minutes, until the sauce is hot and bubbling. Add the butter, if using, and stir until it is fully melted and incorporated.

Step 6

Serve immediately.