Framing a new basement window in cinder block
Option 1 - tapcon screws (or similar) into the solid parts of the block. 

Option 2 - stuff some wire mesh a few inches down the holes. Mix a very stiff mortar/concrete, place on top of the mesh, make a lovely smooth flat surface, and drop in some anchor bolts (probably overkill for a windowframe) or just screw the board in with tapcons or the like but without having to hit the webs since you'll now have solid masonry.

Option 2.5 (modern) - sprayfoam (great stuff or similar polyurethane foam) the holes, then put your concrete/mortar on top of the sprayfoam.