1. To make the cake: Heat oven to 350 degrees F. Cover a Bundt pan with non-stick baking spray.
2. Whisk together flour and baking soda; set aside.
3. In a large saucepan on low heat, stir together ginger beer, butter and unsweetened cocoa until well combined. Stir in rum and sugar, whisk to blend.
4. In a small bowl whisk together buttermilk, eggs and vanilla until well combined. Stir in the ginger beer and rum mixture. Add the flour mixture and stir until no flour streaks remain.
5. Pour batter into prepared Bundt pan and bake until cake tester inserted in the thickest part of the cake is part of the cake comes free of wet batter (a few crumbs will be visible), about 45 to 50 minutes. In the meantime, make the glaze.

To make the glaze:

1. In a small saucepan over medium-high heat, combine the butter, sugar, rum and water. Cook and stir until mixture comes to a boil. Lower heat and let simmer for 5 minutes, then remove from heat.
2. Once the cake is done, transfer the Bundt pan to a wire rack to cool for 5 minutes. Invert pan and turn out the cake out onto serving plate.
3. Use a skewer and poke holes all over the cake and brush the cake all over with the glaze. Serve warm. Cover cake with plastic wrap and store it in the refrigerator for up to 3 days, if any leftovers remain.