In a large mixing bowl, cream together the vegetable oil and brown sugar. Once smooth, add the vanilla and lightly beaten eggs and mix until smooth.
Intermittently add the dry mixture to the oil and sugar mixture while beating, until combined. Take care not to over-mix.
In a small mixing bowl, combine all of the dry ingredients.
Using a spatula, fold the pumpkin puree into the batter until it's evenly combined.
Line a light-colored baking sheet with parchment paper and measure 1 Tbsp amounts of batter onto the pan, taking care to provide enough space between the cakes so that they don't stick to each other. Bake at 350ºF for 12 minutes.
After 12 minutes, remove the whoopie pie tops from the oven. Allow them to sit for 1 minute and then remove them from the pan and place on a cooling rack.
Continue baking additional cookie sheets of whoopie pie cakes until all of the batter is used.
While the whoopie pie cakes are baking, make the frosting. Cream together butter and salt. Then cream in vanilla. Intermittently add confectioner's sugar while continuing to beat until smooth. Next, cream in the milk, and then the flour. For the last step, add the Snap Liqueur. If you are planning to give some of your whoopie pies to children, set aside some booze-free frosting for them.
Once the whoopie pies are cool, make sandwiches by spreading icing in between two cakes.
For best storage, wrap each cake in plastic wrap and store them in the refrigerator for up to a week.