Heat oven to 350 F. Coat a 10-cup/10-inch bundt pan with cooking spray.

In a large saucepan, over medium high heat, melt butter with the water. Remove from pan and pour into a large bowl. Stir in the cocoa and sugar. Pour in the rum and stir to combine. Add the buttermilk, eggs, and vanilla, and stir to combine. Stir chocolate chips into the batter.

Sift the dry ingredients (flour, baking soda, and salt into the wet ingredients). Stir the flour mixture into the wet ingredients until no white streaks remain in the cake batter.

Pour batter into the prepared bundt pan. Bake cake until cake tester inserted into the thickest part of the cake comes out dry (45 to 50 minutes).

In the meantime, make the glaze.

To make the rum glaze:

In a small saucepan, over medium high heat, combine the butter, sugar, rum, and water. Cook and stir until the mixture comes to a boil, then reduce heat and simmer 5 minutes. Remove from heat.

Once the cake is done, let cake cool in cake pan for 5 minutes. While the cake cools, use a skewer to poke holes in the bottom of the cake and brush with glaze. Flip cake onto a wire rack. Use a skewer to poke holes all over the top of cake. Use a pastry brush to brush the cake, while still warm, with the glaze. Let the glaze harden, then remove cake to a serving plate. Serve at room temperature.