For my 18th birthday, my mother baked me such a cake.
A 9"x18" pan produced the body. Mom cut out two triangles from each side to give the lady an “hourglass” figure. The two cutouts were used to give the lady a neck.
The “breasts” were baked in two small stainless steel mixing bowls. Mom said they cooked fine. When the cake rises, you end up with a bulge above the rim of the mixing bowl. Cut off this bulge so the “breasts” will lie flush with the “torso”.
For giggles, you can “pierce” one of the nipples by placing a sliver or gold dragee on either side of the nipple to mimic a barbell.
My team and I made boobie cookies as part of our fundraising for a 60K breast cancer walk last year (we used custom molds from an adult store, since it’s hard to find a cookie-sized option… but a bowl will work wonderfully for cake). Everyone loved the whole concept, but the “pierced” boobies got the best reaction.
Another option is to bake a sheet cake (shape like a body), use some larger cupcakes for the boobies. If you’re not up to icing it and worrying about it getting smooth, you can use fondant and it should be easy to smooth over the chest. It works a lot like playdough and if it’s a little stiff you can nuke it in the microwave for a couple of seconds. You can even cut out circles for the nipples or cover a gumdrop for something extra interesting!