Preheat your oven to 350°F (175°C) and prepare a 12-cup muffin tin by lining each well with a paper liner.

In a large bowl, combine the all-purpose flour, cocoa powder, baking powder, baking soda, and salt. Whisk together until well blended.

In a separate bowl, whisk together the egg, milk, vegetable oil, and vanilla extract until blended and glossy.

Carefully fold the wet ingredients into the dry mixture until just smooth.

Dissolve the instant espresso powder in the boiling water and whisk it into the batter.

Divide the batter among the lined muffin cups, filling each about two-thirds full.

Bake for 16 to 18 minutes; check at 16 minutes with a toothpick.

Allow cupcakes to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

For the frosting, beat the softened salted butter until creamy, then add the cocoa powder and half the powdered sugar along with a splash of heavy cream.

For the caramel sauce, place the granulated sugar in a dry saucepan over medium heat and stir gently until melted into a golden caramel.

Remove from heat and whisk in the diced butter, then slowly pour in the heavy cream while whisking.

Pipe a generous swirl of chocolate frosting onto each cooled cupcake and create a hollow in the center for the caramel.

Spoon a small amount of the caramel into each frosting circle.