Position a rack in the center of the oven and preheat to 325 degrees F. Lightly butter 18 oven-safe espresso cups and put them on a baking sheet.
Put 2 tablespoons of the instant espresso powder in a large mixing bowl and stir in 1/4 cup hot water. Stir until the espresso powder is dissolved. Reserve.
In a large mixing bowl, use a hand mixer to beat together the butter and sugar at medium-high speed until pale and fluffy, 2 to 3 minutes. Add the vanilla extract and egg and beat until well combined.
Sift together the flour, cocoa powder, baking soda, baking powder and salt in a large mixing bowl.
In the bowl that has the dissolved espresso, whisk in the buttermilk and 1/4 cup of the prepared hot fudge sauce. At low speed, add the flour mixture to the creamed butter mixture in three additions, alternating with the espresso mixture. Begin and end with the flour mixture and mix until just combined.
Spoon the batter into the prepared espresso cups, filling each about halfway full (about 2 tablespoons per espresso cup). Bake until a cake tester inserted into the center of a cupcake comes out clean, 18 to 20 minutes. Set the baking sheet onto a cooling rack and cool completely.
Meanwhile, place the heavy cream in a large mixing bowl and use a hand mixer to whip until it holds soft peaks. Refrigerate until ready to use.
Dissolve the remaining 1 tablespoon instant espresso in a small mixing bowl with 2 tablespoons hot water. Whisk in the remaining 1/4 cup hot fudge sauce. Spread evenly onto the cooled cupcakes, about 1 teaspoon per cupcake. Cover the top of each cupcake with whipped cream and a light dusting of cocoa powder, and garnish with a whole chocolate-covered espresso bean.