Preheat the oven to 350°F (175°C).

Sift together the flour, baking powder, salt, and 1/4 cup cocoa powder into a large bowl.

Stir in the ground hazelnuts, confectioners sugar, and chopped semisweet chocolate.

In a separate bowl, whisk together the melted butter, egg, milk, and vanilla extract.

Pour the wet ingredients into the dry ingredients and stir until well combined.

Spoon the mixture into a buttered baking dish.

In a small bowl, mix the brown sugar and 1 1/4 cups cocoa powder.

Sprinkle the brown sugar and cocoa mixture evenly over the top of the pudding mixture in the dish.

Bring 2 1/2 cups of water to a boil.

Carefully pour the boiling water over the pudding mixture in the dish.

Place the dish in the preheated oven and bake for 45-50 minutes, or until the pudding feels fairly firm and springy to the touch.

If using a shallow dish, bake for 35-40 minutes.

Remove from the oven and let cool slightly.

Serve immediately, spooning from the dish to ensure everyone gets both sauce and sponge.