Mix (on medium speed) first set of ingredients for cookies in large bowl. I mix wet ingredients, then add the dry. Becare in adding the hot coffee, it may splatter.
Separate medium bowl mix (slow to medium speed) filling ingredients. I add a cup at a time of the powdered sugar to lessen the chances of making a mess.
Use a medium sized cookie scoop and use a cookie sheet lined with teflon or parchment paper and bake for approximately 10 minutes at 350 degrees. Cool and take spatula to add filling and top with another cookie. I normally wrap each whoopie pie individually with plastic wrap to keep fresh.