Preheat the oven to 325°F
Sift together the flours, pudding mix, baking powder, and salt in a medium bowl.
Combine the buttermilk, dark rum, and vanilla and a small bowl and set aside.
In a large mixing bowl with a wooden spoon, cream the butter while gradually incorporating the sugar. Once all the sugar is incorporated in the mixture has increased slightly in volume, add the eggs one at a time. Wait until each egg has been fully incorporated before adding the next.
Add the buttermilk mixture to the creamed butter mixture, and then add the dry sifted ingredients, and mix for 1 to 2 minutes until fully incorporated.
Butter the inside of a 10 cup Bundt pan, making sure to get all of the nooks and crannies, and then lightly dusted the buttered pan with flour, discarding any excess.
Arrange the pecans in the bottom of the pan and then gently pour the batter over the nuts. Do not stir or tap the pan because it will move the nuts around.
Bake the cake for 45 to 50 minutes, or until a wooden skewer inserted in the center comes out clean. Remove the cake from the oven and allow it to cool in the pan for 10 to 15 minutes.
While the cake is baking, make the glaze. In a small pan over medium heat, melt the butter. Stir in the sugar and water, bring to a simmer, and simmer for five minutes. Add the rum, start to incorporate, remove the pan from the heat, and set aside.
Once the cake has cooled, gently turn it out from the pan onto a cake plate. Use a wooden skewer to pierce the cake all over and then slowly pour the glaze all over it.
Return the glazed cake to the pan and leave it there for at least three hours before turning it out onto the cake plate again to serve it. The reason why you store the cake this way in the pan is because this is a very moist cake and plastic wrap will pull the glaze off of it. The flip-flopping in the pan also serves to evenly distribute the liquid in the cake.