Cut peppers in half and place on a greased sheet pan. Roast peppers in the oven 15-25 minutes until soft and slightly blistered. Let peppers cool slightly before using.
Place evaporated milk, roasted peppers, garlic and bouillon in blender; cover. Blend to desired consistency (chunky or smooth sauce).
Melt butter in medium saucepan over medium heat. Stir in flour, stirring constantly until smooth. Gradually stir in poblano mixture. Cook over medium heat, stirring constantly, until mixture comes just to a boil and thickens. Makes about 2 cups.
To serve, cooked chicken breasts can be placed on individual plates or all on serving platter. Spoon sauce over chicken breasts. If any sauce is remaining, place in bowl or small pitcher for all to enjoy.