Preheat oven to 325 F. Grease a 9x5-inch loaf pan. Set aside.
In a medium bowl, sift flour, baking powder, cinnamon, nutmeg, baking soda, and salt.
In a large bowl beat butter, sugars, and vanilla until light and fluffy.
Blend in sweet potatoes; then eggs, one at time. With mixer on low, add flour mixture in thirds; blend just enough to combine.
Spoon batter into a prepared loaf pan.
Bake until a toothpick inserted into the center comes out clean, about 60-75 minutes. Transfer pan to a wire rack to cool, about 15 minutes.
Remove cake from pan to cool completely.
To make the glaze, add butter to a heavy saucepan.
Cook butter over medium heat until it starts to foam, stirring frequently. Stop stirring, and let it continue to cook until amber brown.
Remove from heat, and stir in powdered sugar and enough milk to make glaze desired consistency. If your powdered sugar is lumpy, sifting it will help make the glaze smoother.
Drizzle over cooled bread. Top with chopped pecans, if desired.