Turn on the oven to 190°C. Pour the butter into a medium size round or square baking pan. Pour pan into the oven.
Beat the eggs with the vegetable oil, add the milk and the sugar. Add the flour. Whisk to combine well. Add the sliced bananas, the raisins, the pistachios and the chocolate. Stir with a wooden spoon.
Remove the pan from the oven (the butter must be already dissolved) and pour batter into the pan. Return into the oven. Bake about 30-35 minutes.
Remove from the oven and cool almost completely. Slice, dust with confectioners' sugar and serve.