Position a rack in the upper third of the oven; preheat to 425 degrees F. Cook the bacon in a medium ovenproof nonstick skillet over medium-high heat, turning, until crisp, 8 minutes. Remove to a paper towel lined-plate with a slotted spoon, reserving 1 tablespoon drippings in the skillet. Let cool slightly, then chop.
Meanwhile, soak the shallots in the vinegar in a small bowl. Puree the avocado, buttermilk, tarragon and 1/2 teaspoon salt in a blender; set aside.
Season the chicken with salt and pepper. Heat the reserved bacon drippings in the skillet over medium-high heat. Add the chicken and cook until browned, about 2 1/2 minutes per side. Transfer the skillet to the oven and bake until the chicken is cooked through, 6 to 8 minutes. Transfer the chicken to a cutting board; chop.
Combine the spinach, grapes and the shallots in vinegar in a large bowl. Add the avocado dressing and toss to coat; season with salt and pepper. Top each serving of salad with chicken, bacon and pepitas.