In a mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

Add the egg and milk to the dry ingredients, whisking continuously until the batter is smooth and free of lumps.

Allow the batter to rest for at least 15 minutes; this helps the leavening agents activate to create the signature bubbles.

Heat a non-stick skillet or frying pan over low-medium heat and brush lightly with a small amount of butter.

Pour about half of the batter into the center of the pan, using the back of a ladle to swirl the batter up the sides of the pan to create a thin, crispy edge.

Cover the pan with a lid and cook for 3 to 4 minutes, or until the surface is full of popped bubbles and looks relatively dry.

Remove the lid and dot the surface with half of the remaining butter.

Spread the chocolate hazelnut spread gently over the pancake and arrange the banana slices on top.

Using a spatula, carefully fold the pancake in half to enclose the filling.

Transfer to a cutting board, slice into wedges, and serve warm.