Preheat oven to 350 F. Butter or grease a large cookie sheet or line with parchment paper. Set aside.
In a medium bowl, stir together flour, salt and baking soda; set aside.
In a large bowl, cream butter and sugars together.
Add in dry pudding powder and cake mix and beat until well blended.
Add eggs and vanilla and mix until smooth.
With mixer on low speed, or while stirring by hand, add flour mixture slowly until well incorporated. The batter will be quite thick. Stir in sprinkles, just until evenly distributed.
Roll into 1" balls and place on prepared baking sheet. Bake at 350 F for 10-12 minutes, just until cookies are lightly golden around the edges and on the bottom. Don't cook too long. My cookies were done at about 11 minutes.
Allow cookies to sit on pan for 1-2 minutes then move to cooling rack. When the cookies first come out of the oven they are sort of puffy and rounded. As they cool, the center flattens out somewhat and the cookies look more like cookies instead of looking like they're pregnant. :)