Using a stand or hand mixer, cream together the softened butter, granulated sugar, and brown sugar until smooth and creamy.
Add in the eggs and vanilla extract and beat until the texture is light and fluffy (it should look much paler than before).
Add in the flour, baking soda, and salt and mix until thoroughly combined. The dough should be wet and sticky.
With a wooden spoon or spatula, fold in the rainbow sprinkles.
Cover the dough and chill in the fridge overnight (minimum of 2 hours if you just can't wait).
Preheat your oven to 350°F and line a light colored baking sheet with parchment paper.
Spoon a generous 3 tablespoons of dough and roll into balls. Place dough balls at least 2-3 inches apart on the baking sheet (they will spread, I usually only do 6 cookies on a half sheet pan).
Bake cookies for 9-11 minute or until the edges are set and the centers are still soft.
Remove from the oven and let rest on the baking sheet for 1-2 minutes, then transfer to a cooling rack.
Notes
Store cookies in an airtight container at room temperature for up to a week (they'll definitely be gone before then though). The dough will keep in the fridge for up to a week (which is great for just making a couple of warm cookies at a time) just make sure to transfer to an airtight container so it doesn't dry out. Freeze pre-rolled cookie dough balls in the freezer for up to 6 months.