According to Wikipedia...
While considered a New England phenomenon and a Pennsylvania Amish tradition, they are increasingly sold throughout the United States. According to food historians, Amish women would bake these desserts (known as hucklebucks, or creamy turtles at the time) and put them in farmers' lunch pails or lunch boxes. When farmers would find the treats in their lunch, they would shout "Whoopie!" It is thought that the original Whoopie pies may have been made from cake batter leftovers.
The whoopie pie is the official state treat of Maine (not to be confused with the official state dessert, which is blueberry pie).
The world's largest whoopie pie was created in South Portland, Maine on March 26, 2011, weighing in at 1,062 pounds. Pieces of the giant whoopie pie were sold and the money was used to send Maine-made whoopie pies to soldiers serving overseas. The previous record holder, from Pennsylvania, weighed 200 pounds.
Origin controversyPennsylvania, Maine, Massachusetts and New Hampshire all claim to be the birthplace of the Whoopie pie. In 2011, the Maine State Legislature considered naming the whoopie pie the official state pie. The proposal received bipartisan support. L.D. 71, officially known as "An Act to Designate the Whoopie Pie as the State Dessert", read "The whoopie pie, a baked good made of two chocolate cakes with a creamy frosting between them, is the official state dessert". The Pennsylvania Dutch Convention & Visitors Bureau and other observers in Lancaster County, PA, note that the whoopie pie comes from the area's Amish and Pennsylvania German culture – origins that are unlikely to leave an official paper trail – and has been handed down through generations. Most likely, Amish groups that migrated to Maine brought the treat to the people of Maine. Mainers assert that Labadie's in Lewiston, Maine has been making the confectionery since 1925.
The now-defunct Berwick Cake Company of Roxbury, Massachusetts was selling "Whoopee Pies" as early as the 1920s, but officially branded the Whoopee Pie in 1928 to great success. It is also claimed that the whoopie pie originated in Massachusetts and spread both north and south. It appears though that German immigrants brought the predecessor of the Whoopie Pie to communities throughout the northeast.