Casting a countertop in place lets you skip both the heavy lifting and the "upside down and backward" thinking involved in precasting into a mirror-image mold. But troweling the surface smooth does require skill. Practice on other projects first.
The countertop rests on a base of ½-inch-thick cement board, enabling you to keep the concrete 1 inch thick except at the edges, which are just deep enough to hide the cement board. The result is a normal-height countertop that's easily supported by standard cabinets.