Preheat your oven to 375 degrees (and make sure that you have a middle rack clear for putting in your whoopie pies). Prepare a cookie sheet (or two, if you prefer) with parchment paper.

Combine flour, cocoa powder, baking soda, and salt. Set these aside.

In a mixing bowl, beat butter, shortening, and brown sugar together on a low speed until they are just combined.

Adjust power to medium and beat for about 3 minutes, or until fluffy and smooth.

Add in the eggs and vanilla, then beat for an additional 2 minutes.

Add in half of the flour mixture, as well as half of the milk, to the butter/sugar mixture. Mix and then scrape the sides of the bowl. Then, add in the other half of the flour and other half of the milk, and combine completely.

With a spoon or cookie scoop (that's what I used and it made it so easy), drop 1 tablespoon of batter on the parchment paper. Space these out at least two inches.

Bake the cakes for approximately 10 minutes each, or until the cakes bounce back when you gently press them. Once you take them out, let them sit on the pan for five minutes before moving them onto a cooling rack.

After the cakes have cooled, you can use a decorating bag (or spoon, if you don't have a bag or other decorating-type item) to add frosting (I used my vanilla frosting recipe) to the flat side of one of the cakes, and then sandwich it with another cake.