Bring stock to a simmer in a 2-qt. saucepan over medium heat; keep warm. Heat 6 tbsp. butter and the prosciutto in a 6-qt. saucepan over medium-high heat. Cook, stirring occasionally, until browned, 3–5 minutes. Add shallots; cook until soft, 4–5 minutes. Stir in rice; cook until translucent, 4–6 minutes. Add wine and bring to a boil; cook until reduced by half, about 1 minute. Add 1⁄2 cup warm stock; cook, stirring, until absorbed, about 2 minutes. Continue adding stock 1⁄2 cup at a time and cooking until absorbed before adding more until rice is tender and creamy, 20–22 minutes. Stir in cream, parmesan, salt, and 1 tbsp. piment d'Espelette; keep warm.
Melt remaining butter in a 12" skillet over medium-high heat. Season scallops with salt and remaining piment d'Espelette. Cook scallops, flipping once, until browned and cooked through, 2–3 minutes. Stir grapefruit zest and juice into risotto and divide between 6 bowls. Top with scallops; garnish with grapefruit segments and tarragon.