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.