I had the same leak and put off fixing it for the better part of a year because all the DIY's you run across are for an alternator mounting bracket gasket. Those poor guys that don't have an oil cooler have the same engine block so BMW made the alternator bracket to include a blanking plate to cover the oil cooler line holes with the same gasket. To make it worse, one of the bolts on the bracket is behind the engine mount so you have to lift the engine and remove the motor mount to access it.

For those of use with oil coolers, the gasket is between the oil thermostat and the engine block. The thermostat itself is attached to the block with 2x e-torx bolts. The lower is slightly obstructed and a u-joint is too long but you can get to it with an e-torx socket and a wobble extension. I replaced both bolts with SS socket head cap screws from Orchard Supply that made re-assembly much easier with a ball head hex socket.

Just off the top of my head you'll have to remove:
- air snorkels
- fan assembly
- serpentine belt
- bracket holding power steering line in front of engine and move out of way
- alternator
- follow oil lines with your hand down to thermostat housing, remove the center bolt and the oil lines will come off
- two e-torx bolts holding housing to block

The biggest PITA is that there is no way to see the thermostat housing and you have to do it all by feel.