Mast step failure

I need to rethink my mast step - or work out what I did wrong the first time. Here's a photo looking down into the boat at what's left of the mast step. There used to be a square block with a hole in the middle, where the paint is missing. It came unstuck, the two 12mm belaying pins held it all together for a surprising length of time, and then the block split.

Here's the remains of the glue joint, it looks like the wood had been coated in wax or oil because the epoxy hadn't stuck very well.

Lashing the mast to the beam wasn't really satisfactory either, as the lashing stretched the mast would lean to leeward, and I think that added to the forces turning the boat into the wind. I plan to make a solid mast partner, glued to the forward bulkhead, and extend the mast about 6" so I can move the mast foot down to the chine battens.