Layout Jig

Use a layout jig to scribe a line across the top
Layout jigs have a hole drilled every 1/2″
Use a layout jig to scribe a line on both sides
Using a chainsaw Cut along the line about 4″ deep
Cut deeper about 8″
Cut again at 14″
I use a 14″ bar for this, a 16″ bar would be even better
From the other side start with the tip down in the channel you cut previously.
Keep the tip angled down as you cut slowly
The 2 slabs will separate leaving an almost perfect surface.