Butter sides and bottom of the loaf pan and lightly dust it with flour; set it aside.
In the bowl of an electric mixer, beat the butter on medium-high speed for about 3 to 4 minutes, or until soft. Stir in the sugar and keep beating until fluffy. Beat in the eggs until well mixed.
In a large bowl, sift together the flour, baking powder, baking soda and salt. Lower the speed on the mixer. Alternate between adding the plantains and the sifted dry ingredient mixture.
Add the vanilla and pecans and mix until thoroughly combined.
Pour the batter into the prepared loaf pan and gently place a piece of aluminum foil on top. Place the pan in the oven and cook for 40 minutes. Remove the foil and cook for an additional 15 to 18 minutes, or until the top of the bread looks golden brown and puffed-out. If you inset a toothpick, it should come out moist but not wet.