I made your given expression a tag, then created another expression to do the grouping. Feel free to combine them into one expression if you'd like.
I was able to write this from an example in the thread that Phil linked to in your original thread.
forEach(
groupBy({[default]Testing/Dataset},it()['classification']),
asMap(
'classification',it()[0],
'TotalMinutes', sum(it()[1],'TotalMinutes')
)
)