For the cookies: Position oven racks in the top and bottom thirds of the oven and preheat to 350 degrees F. Coat 2 baking sheets with cooking spray.
Beat the cake mix, butter, eggs and 1/3 cup water with an electric mixer on medium speed in a large bowl until smooth and no bits of dry cake mix are left. Drop the batter by the tablespoonful (a small ice cream scoop will make this task easier) onto the prepared baking sheets about 1 1/2 inches apart. (You will have additional batter for a second batch.)
Bake until the cookies are set around the edges but still soft in the center, about 8 minutes. Let the cookies cool on the baking sheets for 10 minutes, then transfer them to a wire rack to cool completely (the rounded tops will sink slightly). When the baking sheets are cool, repeat with the remaining batter.
For the frosting: Melt the butter and chocolate in a medium microwavable bowl in the microwave in 1-minute increments, stirring in between, until smooth.
Beat the cream cheese, confectioners' sugar and vanilla together with an electric mixer on medium speed in a large bowl until smooth. Pour the chocolate mixture into the cream cheese mixture and beat until smooth.
Frost the cookies, decorate with sprinkles and serve.
Some cake mix brands will yield flatter and fudgier cookies. Others will produce puffier and slightly cakier cookies. Both are delicious in this recipe.
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