SOLUTION: Rolling back to Unreal 4.22.3
DETAILS:
Investigation reveals that this is an issue with #include headers not finding two files:
UBT_COMPILED_PLATFORM/UBT_COMPILED_PLATFORMPlatformTime.h
UBT_COMPILED_PLATFORM/UBT_COMPILED_PLATFORMPlatformCompilerPreSetup.h
may be related to issue UE-87619
Which led me to this post on the gamedev forums
I was running:
Windows Version 10.0.18362 Build 18362
Unreal 4.25.1
VScode version 1.46
Rolling back to unreal 4.22.3, deleting the BullCowGame files and re-installing them, then restarting my machine, and re-launcing BullCowGame in Unreal 4.22.3 resolved the issue