It's called a blind mitered finger joint. It's cut like a typical box joint, but leave the material on the outside fingers fully intact. After all the fingers are cut, miter the outside fingers.
It's a little tricky to get the miter to fit, because you really can't test fit until the miters are cut, so cut them heavy and nibble away at the miters equally until you get the perfect fit.
Good luck, and have fun! I've been building a LOT of cabinets lately, and am hoping to have the time to get back into real wood joinery soon. I miss the challenge!
