Heat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners. Whisk the flour, sugar, cocoa powder, baking soda, baking powder, and fine salt together in a large bowl.

In a separate bowl, whisk the room-temperature egg, buttermilk, vegetable oil, and vanilla until smooth. Add the wet mixture to the dry ingredients and stir just until no dry streaks remain.

Stir the espresso powder into the hot coffee, if using, and gradually whisk the hot liquid into the batter. The batter will be thin, which is expected for this moist chocolate cupcake formula.

Divide the batter evenly among the liners, filling each about two-thirds full. Bake for 18–22 minutes, until the tops spring back lightly and a toothpick shows a few moist crumbs.

Cool the cupcakes in the pan for about 5 minutes, then transfer them to a wire rack to cool completely. Do not fill or frost warm cupcakes because the caramel and buttercream can become runny.

For the caramel, cook the sugar in a dry saucepan over medium heat until melted and amber in color. Carefully whisk in the butter, slowly add the warmed cream while standing back from the bubbling mixture, then stir in the salt and optional vanilla.

Cool the caramel until it is thick but still spreadable, reserving some for the cupcake centers and final drizzle. If it becomes too firm, warm it very gently until it loosens.

Beat the softened butter until smooth and pale, then gradually add the sifted confectioners’ sugar. Beat in the cooled caramel and vanilla, adding heavy cream or milk as needed until the frosting reaches a pipeable consistency.

Cut a small cone from the center of each cooled cupcake and spoon in a little salted caramel, replacing the cake cap if desired. Pipe the caramel buttercream over each cupcake, drizzle with extra caramel, and finish with flaky salt and espresso beans or chopped chocolate.