Bring rum almost to a simmer in a small saucepan. Remove from heat; add raisins, cover, and let cool. Drain and blot dry.
Melt chocolate and butter in microwave, 1 minute, 20 seconds. Stir until smooth, then stir in coarse salt, cornflakes, and raisins. Drop mixture onto parchment-lined baking sheets in 1 1/4-inch mounds. Sprinkle with flaky salt. Chill until set, about 30 minutes or up to 5 days.