Brush a 9-by-5-inch loaf pan with oil and line with parchment, leaving a 2-inch overhang on long sides.
In a food processor, pulse 225g nuts, dates, peanut butter, vanilla, and salt until finely chopped and mixture holds together. Transfer to a parchment-lined baking sheet and lightly knead to pack together; pat into an 8-inch square. Refrigerate until firm, 20 minutes.
Meanwhile, melt chocolate with oil. Halve nut mixture and press one part into loaf pan. Drizzle with half of chocolate. Refrigerate until chocolate just begins to set, 10 minutes.
Press in remaining nut mixture and spread with remaining chocolate. Sprinkle with remaining nuts and flaky salt. Refrigerate until firm, at least 2 hours. To serve, let stand at room temperature 10 minutes, then remove from pan using parchment before slicing.
