Preheat oven to 350 F (176 C) and line two 12 cupcake baking pans with paper liners.
In a large bowl hand whisk together sifted flour and cocoa powder with salt, baking soda, and espresso powder. Set aside.
Add coconut rum and butter in a medium skillet on medium heat and cook until butter has melted. Stir together until creamy, remove from heat and let cool to room temperature.
In the bowl of a stand mixer or using a large bowl with hand mixer, beat together eggs, granulated sugar, brown sugar, buttermilk, and vanilla until smooth using paddle attachment.
Add the cooled butter to the mixing bowl and blend until incorporated.
Slowly add the cocoa powder and flour mixture to the wet ingredients in stand mixing bowl, while on medium speed. Scrape sides and bottom of bowl as needed.
Once a rich chocolate batter is formed transfer to cupcake liners.
Use a 1.5-2 Tbsp cookie scoop to equally measure each cupcake or simply fill each cupcake liner ¾ full with batter.
Place trays on middle rack of oven and bake for 20-22 minutes.
Cupcakes will dome on top with a slight crack, should spring back when slightly touched with finger tip, or check for doneness by inserting a toothpick that comes out with clean with a few crumbs.
Don't over bake your cupcakes, these are meant to be soft and moist.
Place cupcake pans on cooling rack for 5-10 minutes to cool before removing liners from pan.
Let cupcakes completely cool before frosting them.