Peel the bananas, then cut into chunks about 5 cm (2 in) long. Push a cocktail stick into each chunk. Line a large baking sheet with parchment paper and arrange the banana chunks on top. Place in the freezer for 1 hour until firm.
Place the chocolate and butter in a heatproof bowl set over a pan of simmering water and leave for about 5 minutes until melted, then stir well to combine. Spread out the nuts on a large tray or plate.
Remove the bananas from the freezer and dip them into the melted chocolate mixture, making sure that each chunk is completely coated. Quickly roll them in the chopped nuts to cover completely. Place the coated bananas back on the lined baking sheet and return to the freezer for about 30 minutes to harden. Arrange on a platter to serve.