Preheat the oven to 350°F Grease and flour a 10-inch tube or Bundt pan. (If your pan is nonstick, you can get away with just some cooking spray; no need to flour.).
In a medium bowl, combine the flour, baking powder, baking soda, nutmeg, and salt. Whisk well. In a small bowl or measuring cup, combine the milk and vanilla.
In a large bowl, beat the butter, sugar, and light brown sugar until light and fluffy, stopping once or twice to scrape down the sides of the bowl. Add the eggs one at a time, beating well after each addition. Add the sweet potatoes, and mix until the batter is combined. (The batter may look terrible at this point: curdled, weird, terrible. Don’t worry.).
With the mixer on low speed, add half of the flour mixture. Beat to just incorporate. Then add half of the milk mixture, and continue to beat on low until well blended. Add the remaining flour, followed by the remaining milk, and beat on low until the batter is thick and smooth.
Scrape the batter into the prepared pan, and bake for 60 to 75 minutes, or until the cake springs back when pressed lightly and a toothpick inserted in the center comes out clean.
Cool the cake in the pan on a wire rack for 20 minutes. Run a thin knife around the edge to loosen the cake, and then carefully invert it onto the rack.
Meanwhile, make the glaze, if using. In a medium saucepan, combine the buttermilk, sugar, butter, cornstarch, and baking soda. Place it over medium heat, and bring it just to a gentle boil.
Immediately remove it from the heat, stir well, and set it aside to cool to room temperature. Add the vanilla, and stir well.
Set the wire rack - with the cake atop it - over a rimmed sheet pan. Spoon the glaze through a fine-mesh sieve over the warm cake. (I recommend using a sieve because my batch of glaze had some little gelatinous bits of clumped cornstarch in it.).