Grease a 10-inch Bundt pan with a nonstick spray and dust lightly with flour; set aside. In a medium bowl, combine the flour, baking powder, baking soda and salt. Set aside.
In the bowl of a standing mixer fitted with the paddle, beat the butter and sugar for 3 minutes at medium speed. Lower mixer speed and add eggs, one at a time. Continue to beat for 1 minute.
Alternate adding the flour mixture and buttermilk, ending with the flour. Add vanilla or rum extract. Beat on medium-high speed for 20 seconds. Pour batter into prepared pan, and bake for about 60 minutes, or until a wooden toothpick inserted into the center comes out clean
To make the glaze: In a small saucepan, combine sugar, butter, water and vanilla or rum extract. Do not boil. When the cake comes out of the oven, prick it with a wooden skewer or fork. Pour the glaze slowly over the cake. Let the cake cool completely before removing it from the pan.