For the Cake Pieces 

1. Preheat your oven to 350 degrees Fahrenheit and line baking sheets with parchment paper.
2. In a medium bowl, whisk all purpose flour, baking powder, and salt together.
3. In a large bowl, whisk vegetable oil, granulated sugar, room temperature eggs, vanilla extract, butter or almond extract, and milk together until smooth and combined.
4. Slowly, add flour mixture into the wet ingredients, whisking carefully to make sure that you do not overmix. Only mix until you see all of the dry ingredients disappear into the batter, as overmixing will lead to dry cake pieces.
5. Spoon the batter onto your baking sheets in 1-1/2 tablespoon increments with about 2 inches of space around each circle of batter. Do not scoop bigger than 1 1/2 tablespoons of batter, as the whoopie pie pieces will spread rather than dome.
6. Bake for 8-10 minutes or until a toothpick inserted into the center of the cake pieces comes out with moist crumbs and the tops spring back when poked with your pointer finger.
7. Immediately transfer whoopie pies from the pan to a wire cooling rack or plate. Allow the whoopie pies to cool before filling. This should only take a few minutes, but it is critical to ensuring stable treats.

For the Filling

1. If you are using a hand mixer, bring your butter to room temperature before making the frosting. To do this, simply leave your butter out on the counter 1-2 hours before baking. If you forget, cut your butter stick into 1-tablespoon sized pieces. Microwave the pieces laying flat for 10 seconds before flipping the pieces over and microwaving for another 10 seconds. Repeat the process in 5 second increments as needed until the butter is soft but still cool to the touch. You should be able to make an indent without breaking through the butter piece when sticking your finger in it. It should not be melted whatsoever.
2. In a large bowl with a hand mixer or in the bowl of a stand mixer fitted with the paddle attachment, beat butter on high speed until smooth.
3. Slowly, add powdered sugar while mixing on low speed. The mixture should be smooth.
4. Add marshmallow fluff and mix on high speed for 3 minutes.
5. If desired, transfer filling to a piping bag or a Ziploc bag with the corner cut off to assist with spreading.

For Assembly

1. Once your whoopie pies have cooled, pipe the filling onto half of the whoopie pies. Use a piping bag, Ziploc bag, or knife.
2. Roll the whoopie pies in a bowl of additional sprinkles.
3. Store at room temperature in an airtight container for up to 5 days.