Preheat the oven to 350 degrees Fahrenheit.

In a bowl, combine the all-purpose flour, baking powder, baking soda, and salt.

Set aside.

In a large cast-iron skillet over medium heat, melt the butter.

Add the brown sugar and whisk until the sugar is dissolved, about one minute.

Slowly pour the butter mixture into the flour mixture.

Add the beaten egg, vanilla extract, and espresso powder to the flour mixture.

Stir until well combined.

Fold in the chopped dark baking chocolate and hazelnuts into the batter.

Spoon the batter into the skillet, spreading it evenly.

Bake in the preheated oven for 20 to 25 minutes or until the top is golden and a toothpick inserted into the center comes out clean.