Thanks to Jan for figuring out that the mkdir issue had to do with the Root folder and not the user's VST3 folder. Although the error was coming from the VST3 CMake module, the bad data was coming from my VST3 CMake script which was using variable names from SDK3.6.10 and at some point (3.6.13, I believe) this was changed. 

The ASPiK v1.6.1 update fixes this issue. 

You can patch it for yourself in existing projects by doing the following:

Go to the ASPiK folder: ASPiK_SDK/PluginTemplate/project_source/cmake/vst_cmake

and copy the CMakeLists.txt file over the same file in your project replacing the old script:

[[your_project]]/project_source/cmake/vst_cmake/CMakeLists.txt

Then, re-run cmake to update the Xcode project with the correct post-build script. This will get rid of the mkdir issue. 

- Will