Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the vanilla cake mix, softened butter, egg, and vanilla extract. Mix until a soft dough forms.
Slowly add the milk to the dough while mixing. The dough should be soft and slightly sticky but not too wet.
Gently fold in the semi-sweet chocolate chips and rainbow sprinkles, distributing them evenly throughout the dough.
Using a cookie scoop or tablespoon, drop spoonfuls of dough onto the prepared baking sheet, leaving some space between each cookie.
If desired, gently press down on each cookie with the back of a spoon to slightly flatten them.
Bake the cookies in the preheated oven for 8-10 minutes or until the edges turn golden brown. Be careful not to overbake them to maintain a soft and chewy texture.
Remove the cookies from the oven and allow them to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.