Cookies: In a mixer, combine cake mix, eggs, water and oil. Whip together for 2-3 minutes.
On a baking sheet lined with parchment paper, scoop out batter into round mounds using a small ice cream scoop that measures about 1.5 inches in diameter. Use the back of a spoon to round batter into perfect circles. Bake for 8 minutes. Let cool completely.
Cream Filling
In a mixer with the paddle attachment, whip together butter and sugar, then slowly add powdered sugar until blended.
Whip for 5 minutes until light and fluffy. Scoop into a pastry bag.
To assemble, pipe cream filling onto one cake circle, bottom side up, then top with another cookie (right side up to create a sandwich. Repeat. Refrigerate at least 30 minutes to set, then serve.