Preheat the oven to 350°F and line two large baking sheets with parchment paper and set aside.
In a large bowl with a handheld electric mixer or stand mixer fitted with a paddle attachment, cream together the butter and sweetened condensed milk until smooth.
Mix in the powdered sugar, vanilla extract and salt until fully incorporated.
Add in the flour (1 cup at a time), mixing completely after each addition. Scrape down the sides of the bowl as needed.
Mix in the sprinkles (don’t over mix - just until sprinkles are incorporated into the batter).
For mini cookies, transfer the dough to a piping bag fitted with a Wilton 1A tip and pipe teaspoon-size dollops on the parchment about 1-inch apart, then press down with your finger to flatten.
For regular sized cookies, use a medium cookie scoop to portion out the dough into balls, either dropping directly onto the parchment or rolling between your palms beforehand. Optional: use a glass to press down the cookies before baking.
Note: at this point I will add some extra sprinkles on top of the cookies to make them more colorful but this is optional.
Bake one sheet at a time for even baking. Regular sized cookies bake for 8 to 10 minutes and the mini cookies bake for 6 to 8 minutes.
Transfer cookies to a wire rack and bake off the remaining dough. Allow cookies to cool.
Optional: Whisk together the powdered sugar, milk and vanilla extract until smooth. Then enjoy!
Notes
Be sure to measure flour properly or you'll end up with dough that's too stiff to pipe when making the mini cookies. This is super important. If you cram your measuring cup full then these will not turn out. See my FAQ's above on how to do that.
Depending on which size cookie you choose to make, you could end up with 48 regular sized cookies to 280 mini cookies.
Make sure you use room temperature, soft butter (not melted) for this cookie dough.
Store cookies at room temperature for up to a week (icing must be refrigerated.)