First of all, preheat your oven to 300F. Then butter a 4 1/2-inch-deep (12-cup) bundt pan and dust with cocoa powder.
In a medium pot, melt the butter into pools of deliciousness. Remove from heat and stir in the chocolate pieces. Cover and let melt.
Meanwhile, whisk together flour, baking soda, and salt.
Stir the rum, vanilla extract, coffee, and sugar into the melted butter mixture.
Slowly combine butter mixture with dry mixture (add about half, whisk until smooth, then add remainder). Add 3 eggs and beat until a smooth batter forms. Scrape the sides as needed to work in all the flour.
Pour into bundt pan and bake for 1 hour and 50 minutes. Be sure to leave at least one inch clear at the top of the bundt pan so that it doesn’t overflow.
Let the cake cool to room temperature on a rack, then flip out.
If you rush and flip it out while still warm, half your cake might stay in the pan.
Once flipped, you may refrigerate until needed (for up to 3 days). You can even poke it all over with a skewer and sprinkle on a shot or two of extra rum, if you’re feeling wild (just be sure to let it sit overnight to soak it all up evenly).
Then dust with a flurry of powdered sugar, slice thickly, and serve with a dollop of whipped cream.