For the crepes: Combine the eggs, milk, flour, cocoa powder, sugar and salt in a blender or bowl. Blend or whisk by hand until the mixture forms a smooth batter.
Preheat a 10-inch nonstick crepe pan or skillet over medium-low heat. Melt 1 cube of butter in the pan. Add 1/4 cup of batter and quickly tilt the pan to form an even coating of batter on the bottom of the pan. Cook until set, about 1 minute. Using a heat-resistant spatula, carefully loosen the sides and gently flip the crepe over. Cook for 1 minute. Repeat with the remaining batter and butter.
For the filling: Mix the jam and mascarpone in a small bowl until smooth.
Spread each crepe with 3 tablespoons of the jam mixture leaving a 1/2-inch border. Fold the two opposite ends of each crepe inward and roll into a tube shape. Repeat with the remaining ingredients. Arrange the crepes on a platter.
Heat the chocolate-hazelnut spread over low heat in a small saucepan, stirring constantly, until warm. Spoon over the crepes and serve.