Place an oven rack in the center position, then preheat your oven to 350°F (175°C). Line all 12 cups of your muffin pan with cupcake liners.
In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, salt, and espresso powder for about 30 seconds, until evenly combined with no streaks or clumps.
Make a well in the center of the dry ingredients. Add the egg, buttermilk, vegetable oil, and vanilla extract, whisking the wet ingredients together first, then slowly pulling in the dry ingredients until the batter is thick and mostly combined.
Pour the hot water into the batter in a slow, steady stream while whisking gently. The batter will become very thin and pourable. Scrape the bowl with a spatula to ensure no pockets of dry ingredients remain.
Use a cookie scoop or 1/4 cup measuring cup to portion the batter into the liners, filling each about two thirds full. Wipe away any drips and tap the pan gently on the counter to release air bubbles.
Bake on the center rack for 18 to 22 minutes, checking at 18 minutes with a toothpick. The cupcakes are done when the toothpick comes out clean or with a few moist crumbs, and the tops spring back when lightly pressed.
Let the cupcakes rest in the pan for 5 minutes, then transfer them to a wire cooling rack. Allow them to cool for at least 1 hour, until fully cool to the touch.
Beat the softened butter on medium speed for 2 to 3 minutes until pale and fluffy. Gradually add the powdered sugar and cocoa powder on low speed, then add the vanilla, salt, and 3 tablespoons of heavy cream. Beat on medium-high for 2 minutes until light and spreadable.
Spoon the frosting into a piping bag fitted with a large star tip. Pipe a spiral from the outer edge toward the center of each cooled cupcake, finishing with a gentle lift to make a peak. Sprinkle with mini chocolate chips.