Line a baking sheet with aluminum foil; set an ovenproof wire rack inside baking sheet. Stir together espresso, cocoa, and 2 teaspoons each of the kosher salt and pepper in a small bowl. Sprinkle evenly all over beef. Place on wire rack. Chill, uncovered, at least 8 hours or up to 24 hours.

Remove beef from refrigerator; let stand at room temperature 1 hour. Meanwhile, melt ¼ cup of the butter in a small pot over low. Set aside remaining ¾ cup butter to soften. Add onion and remaining ½ teaspoon kosher salt to melted butter. Cook, stirring occasionally, until deep golden, about 30 minutes. Add sherry and remaining ¼ teaspoon pepper. Cook, stirring often, until sherry has mostly evaporated, about 2 minutes. Transfer to a medium bowl. Cool completely, about 15 minutes.

Cut softened ¾ cup butter into ½-inch pieces. Add to onion mixture, and stir gently until combined. Cover; chill until ready to use. (Onion-butter mixture can be made and chilled up to 3 days in advance. Let stand at room temperature about 20 minutes to soften before using.)

Preheat oven to 400ºF. Heat oil in a large cast-iron skillet over medium-high. Add beef; cook until browned on all sides, about 8 minutes total. Return beef to wire rack on baking sheet. Roast until a thermometer inserted in center of meat registers 115ºF, 25 to 30 minutes. Let stand 30 minutes or up to 1 hour. Thinly slice beef on an angle.

Spread about 1 ½ teaspoons onion-butter mixture onto each baguette slice. (Reserve remaining onion-butter mixture for another use.) Top each slice with 2 or 3 baby arugula leaves and 1 beef slice. Sprinkle with flaky sea salt before serving.