Cook and stir almonds in a skillet over medium-high heat until golden and toasted, 2 to 4 minutes.
Stir sugar, rum, and butter together in a microwave-safe bowl. Cover the bowl with a lid or plastic wrap and microwave until hot, stirring once, 2 to 3 minutes.
Place plantain slices in rum sauce, sprinkle with cinnamon, cover the bowl, and microwave until plantains are hot and tender, 1 to 2 minutes more. Top with toasted almonds.
