Line a small baking tray or chopping board with baking paper.
Peel the bananas and slice each one in half lengthways. Arrange them side by side on the baking paper.
Place another sheet of baking paper on top and gently press down with the base of another baking tray or a chopping board until the bananas form one even layer, about 1cm thick.
Spread the peanut butter evenly over the flattened bananas.
Scatter over half of the frozen raspberries and half of the chopped pistachios, pressing them gently into the peanut butter.
Melt the dark chocolate in a heatproof bowl over a pan of simmering water (or in the microwave in 20-second bursts), stirring until smooth.
Pour the melted chocolate over the peanut butter layer and spread it evenly to cover.
Sprinkle over the remaining raspberries and pistachios while the chocolate is still melted.
Transfer to the freezer for 2–3 hours, or until completely firm.
Remove from the freezer and leave for 2–3 minutes before cutting or breaking into 12 pieces. Store in an airtight container in the freezer for up to 1 month.