Roast the jalapeños directly over a gas flame or under the broiler, turning, until charred all over. Transfer to a small bowl, cover with plastic wrap and let cool. Peel, seed and coarsely chop the jalapeños.
In a blender, puree the jalapeños with the peas and water. Transfer the puree to a medium saucepan.
Cook the puree over moderate heat, stirring occasionally, until piping hot, about 3 minutes. Stir in the butter, season with salt and serve.
The puree can be prepared through Step 2 and refrigerated for 2 days.