Notes: If making ahead, cover and chill up to 1 day.
1. In a bowl, combine chipotles and 2 cups hot water. Let stand until chipotles are soft, 10 to 15 minutes.
2. Pull off and discard chili stems, seeds, and veins. Also discard the soaking water.
3. 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.
4. 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.
Nutritional analysis per tablespoon.