Preheat an oven to 180 degrees C. Grease 16 muffin cups or line with patty cups.
Stir the flour, sugar, polenta, baking powder and salt together in a bowl. Whisk the milk, plantains, eggs, oil and butter together in a separate bowl. Mix the flour mixture into the milk mixture until just combined; do not over mix. Allow the mixture sit at room temperature for 10 to 15 minutes. Pour into prepared muffin cups to about 2/3 full.
Bake in the preheated oven until golden and the tops spring back when lightly pressed, 15 to 18 minutes.