For the cookie sandwiches:
Your favorite boxed cake mix
For the filling:
Sprinkles, miniature chocolate chips or other toppings.
Preheat the oven to 350 degrees F. Use a silicone cooking mat or line baking sheets with parchment paper. Mix the batter according to the box instructions but use 1/3 less water. once combined, spray a coffee measuring scoop (or something to measure out about an 1/8 cup) with nonstick cooking spray. Scoop batter and arrange on mat, leaving a 1 ½-inches between each cake batter clumps. Cook according to the box directions. Place on rack to cool. Assemble by placing desired filling contents on the flat side of a cookie and then placing another cookie on top to form a sandwich. Decorate by gently squeezing assembled sandwich until the filling starts to come out of the sides and rolling in sprinkles or other confections like mini chocolate chips.