Great! I first tried as .txt, but then thought the native Mathematica Package file type would make the most sense. Note that if you wanted to use .wl rather than .m (.wl is the version 10 and future file suffix for packages, though I am not sure why it has changes other than for marketing reasons) you need to explicitly tell Export that it is package format as in
exp = Export["/Users/dreiss/Desktop/dataset.wl", dataset,"Package"];
since Export apparently does not yet know that the .wl suffix is to be interpreted as a Package format as well as .m
To read in this file using Import one would then need also to explicitly specify the "Package" as in
If one just tries
exp = Export["/Users/dreiss/Desktop/dataset.wl", dataset]
one gets an error message and the return is $Failed
I reported this as a bug since Export really should know this.