Couple of substitutions/additions:
1. Used Chobani plain nonfat greek yogurt rather than sour cream. I use that substitution quite a bit to good effect.
2. Couldn't bare the thought of a vanilla looking cake so added a 1/4 cup of unsweetened cocoa powder. Voila! Chocolate cake.
3. Added some instant coffee. About 1/8 cup, for additional flavor.
4. Did not do the streusel mixture at all. Frosting, which I did not make, may be overkill.