In a bowl, combine chipotles and 2 cups hot water. Let stand until chipotles are soft, 10 to 15 minutes.
Pull off and discard chili stems, seeds, and veins. Also discard the soaking water.
Meanwhile, in a 10- to 12-inch frying pan, combine 1 teaspoon butter and onion. Stir often over high heat until onion is limp, about 1 minute. Add chipotles and chicken broth.
Scrape mixture into a blender. Whirl until very smooth. Return to pan and stir over high heat until simmering, about 3 minutes, then add lemon juice and remaining butter. Remove from heat and stir until butter melts. Season to taste with salt. Use hot, warm, or at room temperature.