Preheat oven to 175°C (350°F) with convection setting. Halve the marshmallows. Peel the bananas and cut each in half lengthways.
Melt the butter in a non-stick pan over medium heat. Fry the banana halves in batches for 3 minutes, turning occasionally. Distribute the cooked banana halves on 4 oven-safe plates.
Spread the halved marshmallows on top of the bananas on the plates. Bake in the oven for 5 minutes at 175°C (350°F) convection until the marshmallows are golden brown.
Remove from oven. Top each plate with 2 scoops of vanilla ice cream. Garnish with the finely chopped pistachios and chocolate sauce. Serve immediately.