In a large bowl, beat together the butter, brown sugar, and white sugar. (TIP: The butter should not be melted AT all.) Beat until ingredients are completely smooth and combined, and light and creamy.
Add in the egg and vanilla extract and mix until combined.
In a separate bowl, whisk together the baking soda, salt, unprepared pudding mix, unprepared cake mix, and flour. Stir until combined.
Add the dry ingredients to wet. Beat to JUST combine (do not overmix). The cookie dough gets very thick. Add in the sprinkles and chips and gently fold into the dough with a spatula.
Form tall cookie dough balls that are about 1.6 ounces in size. Cover and chill for at least 1 hour.
Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper or a baking liner.
Place no more than 6 cookie dough balls per cookie sheet to give ample room for the dough to spread while baking.
Bake for 9-13 minutes or until lightly browned at the edges and still soft (slightly under-baked in the center; we bake for about 11 minutes).
Remove from the oven and allow to continue to "cook" on the cookie sheet for another 3-4 minutes. While they are on the cookie sheet I add in a few more chocolate chips and sprinkles to make them look pretty. This is optional. Remove with a metal spatula to a cooling rack until fully cooled. Cookies are best enjoyed the same day they're made.
Store any leftover (completely cooled) cookies in an airtight container at room temperature.