Combine the softened butter, capers, green onion, Parmesan cheese, parsley and vinegar in a bowl. Set aside.
Bring a large pot of salted water to a boil. If potatoes are large, cut into halves or quarters. Add potatoes and cook until tender but still firm, 15 to 20 minutes. Drain.
Add the caper sauce to the pot of drained potatoes and toss gently to coat. Season to taste with salt and pepper.
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