PREHEAT oven to 425°F. Place roast in baking pan; brush with 2 Tbsp. of the steak sauce. Bake 35 to 40 minutes or until internal temperature reaches 140°F for medium-rare or 155°F for medium doneness, brushing with 2 Tbsp. of the remaining steak sauce after 20 minutes. Let stand 15 minutes (internal temperature will rise 10°F during standing).
PLACE potatoes in large saucepan. Add enough water to cover. Bring to boil on medium-high heat. Reduce heat to medium-low; simmer 10 to 12 minutes or until potatoes are tender. Drain potatoes, leaving potatoes in pan. Cut 1/4 cup of the butter into pieces; add to potatoes along with the parsley. Toss tightly until potatoes are evenly coated. Season with the salt and pepper. Cover to keep warm.
FILL bottom of double boiler with a few inches of water. Bring to boil on medium-high heat. Meanwhile, mix remaining 1/4 cup steak sauce and shallots in top of double boiler; place directly on burner (not over the bottom part of the double boiler filled with water) and cook 3 minutes or until thickened, stirring frequently. Add 2 Tbsp. of the remaining butter; stir until melted. Remove from heat. Add egg yolk and tarragon; stir with wire whisk until well blended. Place over bottom of double boiler filled with simmering water; cook and stir until thickened. Add the remaining 6 Tbsp. butter, 2 Tbsp. at a time, stirring with wire whisk after each addition until butter is completely melted. Remove from heat; cover to keep warm. Cut meat into slices. Top with the sauce. Serve with the potatoes.