COOKIES: In a stand mixer (or using a large bowl and electric mixers), combine cake mix, pudding mix, butter, eggs, and vanilla. Beat until you have a very thick dough. Scrape the bowl as needed. Finely chop whole Oreos (with the creme) and fold into the dough.
DOUGH BALLS: Roll 1-1/2 tablespoons of dough (30 grams each on a food scale) into balls. Chill balls for 30 minutes to an hour to keep them puffy during baking.
BAKE: Preheat the oven to 350°F. Line a baking sheet with parchment paper or a liner. Place dough balls 2 inches apart on the sheet. Bake for 9-10 minutes until bottoms are slightly browned. Don’t overbake. Let cookies rest on the sheet for 5 minutes before moving to a cooling rack. Repeat with remaining dough.
FROSTING: Soften the cream cheese in the microwave on a safe plate using 15-20 second bursts until room temperature. In a mixer, cream the butter and cream cheese until smooth. Mix in vanilla and salt. Gradually add powdered sugar, beating until smooth. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 1-2 minutes. For thicker frosting, add an extra 1/4 cup of powdered sugar. Chill the frosting for 20-30 minutes to firm up before frosting.
ASSEMBLE: Once cookies are cool, transfer frosting to a piping bag with a wide tip. Pipe frosting on one cookie and gently sandwich with another. Repeat with all cookies.
SOFTEN COOKIES: Cover whoopie pies with plastic wrap and refrigerate overnight to soften (Note 2). The next day, let them come to room temperature before serving. They are best enjoyed on that same day.
Video
Notes
Note 1: I've tried many different cake mixes, but none match the flavor and texture of this one! Be sure to use a 14.25 oz box. Do not follow directions on box! Note 2: This is where they become soft and cake like, plus more flavorful. If cookies haven't had a chance to thoroughly soften, the frosting squeezes out when you bite into them!