Black Birch 5 years later.

Still alive and produced 3 pounds of Chaga.  Nectria galligena damage with Chaga growing around the edge.
At the widest part the tree is about 2.5 feet in diameter.

Pieces of Chaga on a cutting board.  The piece on the bottom and center right has bark attached.

Inner flesh color.

Inner flesh color where attached to the tree.

Close-up of small piece (1.5" wide) showing marbling.  The white part is not mold.  This kind of "marbling" is common to Chaga that is closest to the surface of the birch.  I suspect that this is the color of the mycelium as it comes out the tree.  Later it turns yellow then brown and on the surface, exposed to the air, it turns black.  When Chaga gets wet the color brown is dissolved and in turn changes the color of the strand that are white or yellow.  This "marbling" is not present in Chaga which is further from the surface of the tree.  The moisture from the rain evens out the color of Chaga that is furthest from the surface of the tree.

Chaga Lookalike
Guill identified the Chaga lookalike shown here:
Here is one other that I found in Grafton.
Here is one that was growing on an American Beech in the Berkshires.