Preheat the oven to 350 degrees F. Butter and flour a 12-cup Bundt pan.
In a small bowl, combine the brown sugar, pecans, and cinnamon; set aside.
In a large bowl, combine the 1 cup of butter and sugar. Beat at medium speed with an electric mixer until creamy. Beat in the eggs, 1 at a time, beating well after each addition. Stir in the sour cream, sweet potatoes, and vanilla.
In a medium bowl, combine the flour, baking powder, cinnamon, salt, and nutmeg. Gradually add it to the butter mixture, stirring well.
Pour half of sweet potato batter into prepared pan. Sprinkle evenly with the brown sugar mixture. Spoon the remaining batter evenly over the streusel. Bake until a tester inserted in center comes out clean, about 1 hour. Remove the cake from the oven and cool. Spoon the Rum Glaze over cooled cake, slice and serve.
In a small bowl, whisk together all of the ingredients until smooth.