You can make this modification with any fairly fitted bodice pattern that has a center back zipper. I started with the McCall's 5845 pattern. These instructions assume that you are familiar with the basic steps of dressmaking and will not teach you to insert the zipper or make the dress; I'm only explaining how to modify your existing dress pattern for the cutout.
Step 1: Prep your bodice pattern.
You may find it helpful to sketch a shape on another piece of paper first, then use it to trace the outline onto your lining fabric.