How to directly access env var in Windows right after setting this var

Question

When in run

SET TEST_VAR=my_var && echo %TEST_VAR%

I get:

%TEST_VAR%

Which mean I cannot access the setted env var in the current process.

How can I get the TEST_VAR var in the same process ?

I need it in the same process because I'm running this code in dart like this:

import 'dart:io';
void main(List<String> args) {
  Process.run(r"SET TEST_VAR=my_var && echo %TEST_VAR%", [], runInShell: true);
}

Ps: it's not the real command but I'm running a third party code which require some env var to be setted.