Preheat an oven to 170 degrees C. Grease a 20cm square tin.
Melt butter in a saucepan over medium-low heat. Remove from heat, add choc chips and stir until melted. Set aside to cool for 5 minutes.
Lightly beat the egg in a medium bowl. Stir in the brown sugar, banana, vanilla and salt. Pour the melted chocolate mixture into the banana mixture and stir until well combined. Add the flour, stirring just until incorporated. Spread the batter into the prepared tin.
Bake in the preheated oven until a toothpick inserted near the centre comes out with moist crumbs, about 30 minutes. Don't over cook. Remove and cool tin on wire rack before cutting.