Preheat oven to 350°F. Cream brown and white sugars with butter until very light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in mashed bananas. Sift flour with baking powder and salt. Mix milk and vanilla together. Add each to first mixture alternately. Stir in the pecans. Pour into a 10 inch well greased tube pan. Bake in preheated oven for 1½ hours, or until firm. Place upside down on a cake rack; when cool, invert and turn out onto rack.