Whisk the cocoa powder and sugar in a large bowl to break up any clumps. Whisk in the flour, baking powder, baking soda and salt.  In a separate bowl, whisk the eggs, buttermilk and melted butter. Gradually fold the wet ingredients into the dry ingredients until just combined. Fold in the chocolate chips. Heat a large skillet or griddle over low to medium-low heat and smear a little bit of butter over the surface. Add 1/3-cup scoops of batter to the pan and cook until small bubbles form on the surface and the edges look dry, 2 to 2 1/2 minutes. Flip and cook until golden brown on the bottom and set in the middle, about 2 more minutes. Repeat with the remaining batter, adding more batter to the pan if necessary.  For the chocolate sauce: Heat the heavy cream in a small saucepan until hot but not boiling. Remove from the heat and add the chopped chocolate, stirring until melted and smooth. Top the pancakes with the chocolate sauce and marshmallows.