I'm glad you asked! In December 1917, the Mistletoe Troop in Oklahoma sold cookies in their local high school.
In 1922 the Girl Scout magazine suggested using cookie sales as a form of fundraising and published a sugar cookie recipe from the Chicago regional director.
From 1933 to 1935 cookie sales became popular with troops in Pennsylvania and New York until eventually, in 1936, Girl Scouts of the USA licensed commercial bakeries to produce the cookies.