For the chicken: Preheat the oven to 375 degrees F.
Slide your fingertips beneath the skin of each chicken breast to loosen. Drizzle each breast with olive oil and sprinkle with salt and pepper, rubbing beneath and on top of the skin.
Place the chicken on a baking sheet or in a baking dish and roast in the oven until the chicken is cooked through, about 35 minutes. Set aside and cover with foil to keep warm.
Meanwhile, for the sauce: Preheat a large, heavy skillet over moderate heat until hot. Toast the pumpkin seeds, stirring constantly, until they have expanded and begin to pop, 3 to 5 minutes. Transfer the seeds to a plate to cool. Reserve 2 tablespoons for garnish.
In a medium, heavy saucepan, simmer the tomatillos, serrano and onions in salted water to cover until the tomatillos turn a dark green color, about 10 minutes. Using a slotted spoon, transfer the tomatillos, serrano and onions to a blender and puree with the chicken broth, cilantro, sugar and toasted pumpkin seeds until smooth (the sauce will be a little coarse). Season with salt and pepper.
To serve, remove the skin from the chicken breasts and slice the chicken crosswise on the bias. Transfer to a serving plate. Spoon the green pipian sauce on top and garnish with the reserved toasted pumpkin seeds.