Preheat oven to 325°F. Butter and flour a bundt pan to prevent the cake from sticking. Tap out any excess flour. (see video)
In a large bowl of an electric mixer or using a hand mixer, beat the eggs, oil, water and rum just until mixed.
Then add in the cake mix, instant pudding, flaxseed and cardamom (if using) and beat until smooth.
Pour the batter into your prepared bundt pan and bake for 55-60 minutes. Cake will have a light golden brown color on top when finished.
Cool cake in the pan for 5-10 minutes.
Make the glaze. Melt the butter in a medium sized saucepan over medium-low heat. Then add the sugar and water and stir. Let this simmer for 1-2 minutes.
Remove from heat and stir in the Bacardi rum. Pour into a 2 cup measuring cup and set aside.
Invert the cake onto a plate or cooling rack. Prick top of cake (you can use a fork, toothpick or skewer) and then slowly pour 1/2 cup of the glaze into the cake. (You will want foil or parchment paper underneath the cake since glaze will run down the sides).
The rum cake will start to absorb the glaze. Add another half cup of the glaze and wait a few more minutes to absorb it. Repeat until glaze is finished (you don't have to use all of it if you don't want to but I always use it all, the cake soaks it all up!).
Sprinkle with powdered sugar or drizzle with melted chocolate and enjoy. Cake is best left at room temperature.