• Preheat your oven to 180 C/ 350 F. Grease an 8 inch or 7 inch pan with butter, dust with flour and line the bottom with parchment.
  
• Mix the cocoa with the boiling water well. Rest until its at room temperature.
  
• Stir or whisk together the flour, sugar , baking soda, baking powder and salt.
  
• Whisk in the cooled cocoa mixture and the egg .
  
• Add the yoghurt, vanilla and oil. Whisk well until smooth.
  
• Pour batter into prepared pan and bake at 350 F/ 180 C for 42-45 minutes for an 8 inch pan and 45-50 minutes for a 7 inch pan. The cake is done when the top springs back when pressed and a toothpick inserted in the centre comes out clean.
  
• Cool in pan for 5 minutes and then unmould , rest on a wire rack until completely cool.
  

For the Nutella dark chocolate ganache

• Chop up the chocolate and add the Nutella in a mixing bowl.
• Heat the cream on the stove top or in microwave until bubbles appear around the sides. Pour the hot cream over the chocolate and Nutella , let stand for 2 minutes.
• Stir until all the chocolate is melted and the nutella, chocolate and cream are well blended. Let rest at room temperature for 4 - 6 hours or until thick set.
• Divide the ganache into two- one for filling and one for frosting. Whip the filling for few minutes if you want it lighter. Crush the Ferrero Rochers and fold into the filling. 

For the assembly

• Slice cool cake into desired number of layers. Chill the cake (and then the layers) to make it easier. Place one layer on a cake board or plate. Spread on about half a cup of the filling ganache (with the crushed Rochers) . Place second layer on top and continue filling and layering all layers .
• Chill for about 20 minutes. Use the frosting ganache to cover sides and top . Decorate as desired. Keep the assembled cake in the refrigerator. Remove half hour before serving.