Spray the 12" Large Skillet with non-stick cooking spray and set aside.
Combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl and whisk to combine.
In another bowl, whisk the buttermilk and espresso powder together until the coffee granules are dissolved.
Whisk the oil and eggs into the buttermilk mixture.
Whisk the buttermilk mixture into the flour mixture until just combined.
Whisk in the melted chocolate until the batter is a uniform color with no streaks of flour or chocolate.
Scrape the batter into the prepared skillet and smooth it out to the edges of the pan.
Cover and cook over low heat until the edges are cooked and the center is set, 45 to 65 minutes. (You will need to gently touch the cake with your fingers. It will feel undercooked in the middle, but it shouldn’t be goopy.)
Replace the cover and move the pan off-heat. Allow to stand for 5 min.
Uncover and let stand for about 5 minutes until the surface of the cake is dry.
Place a cooling rack over the skillet and invert the cake onto the rack.
Serve warm or cool completely. Dust with powdered sugar and serve.