Preheat oven to 375 degrees. In a large skillet or high-sided sauté pan, melt ¼ cup butter over medium-low heat and add breadcrumbs to pan. Toast breadcrumbs, stirring constantly, until deep golden-brown, about 5 minutes. Transfer toasted breadcrumbs to a paper towel-lined plate and wipe any remaining crumbs from the pan.
Add remaining 2 tablespoons butter to pan and sauté shallots over medium heat until soft. Add flour to pan, stir, and cook for 1 minute. Add chicken stock, stirring constantly, and bring to a boil. Reduce heat to low and add butterbeans to pan. Stir to combine and leave pan on heat until beans are just warmed through.
Remove pan from heat, stir in sour cream, and season with salt and pepper. Transfer to a casserole dish, top with toasted breadcrumbs, and bake for 40 minutes or until bubbling.