Preheat the oven to 350 degrees F.
For the Dry Ingredients - in a large mixing bowl, add flour, baking powder, baking soda, granulated sugar and salt. Whisk the ingredients together (whisking has a similar effect to sifting the dry ingredients). Set aside.
For the Egg, Canola Oil and Buttermilk Mixture - In a medium size bowl, add the egg. Then beat the egg, and add the canola oil and buttermilk. Whisk the egg mixture together.
For the Batter - To the mixing bowl with the dry ingredients, add the egg mixture and stir until moistened.
Baking the Whoopie Pies - Brush melted butter over each of the Whoopie pie baking pan cavities.
To each cavity in the Whoopie pie baking pan, add batter 3/4's of the way full. Top the batter with blueberries.
Uses for Extra Batter - The batter should be enough for 16 cavities (if using a 12-cavity baking pan, another batch with 4 cavities can be made which would yield 2 more Whoopie pies). If baking the second batch of 4 cavities, reserve some of the blueberries to add to that batter.
Or another option for using the extra batter is to make mini muffins with it. If making mini muffins, also brush the melted butter over each of the muffin cavities on the muffin tray. An option is to cut 12 thin slices of strawberries and add them to the top of the mini muffins.
Bake the Whoopie pies in a preheated oven until the tops turn a golden color and the edges of the pie start to pull away from the pan (about 12 minutes).
After removing the Whoopie pies from the oven, when the pan and pies are cool enough to handle, remove the pies from the pan onto a cooling rack and let cool.
While the Whoopie Pies and, or muffins are cooling, the frosting filling can be made.