Did you have a piece custom made for the cabinets? If so, the concrete artisan should have templated your area to get an exact fit. Have them remake the countertop for you. If it is just at the windowsill where it is short, you should be able to have a sill piece made to fit, you will have a seam but should be minimal, or you could have it made a little thicker and have the edge lip the counter by a small amount and silcone the seam, might look more finished this way.