Microwave the butter for about 40 seconds. Butter should be completely melted but shouldn't be hot.
In a large bowl, mix butter with the sugars until well-combined.
Stir in vanilla and egg until incorporated.
Add the flour, baking soda, and salt. Please read the recipe note about properly measuring flour.
Mix dough until just combined. Dough should be soft and a little sticky but not overly sticky.
Scoop out 1.5 tablespoons of dough (medium cookie scoop).
Roll the tops (just the tops) in sprinkles, and place 2 inches apart on baking sheet.
Bake for 7-10 minutes, or until cookies are set. They will be puffy and still look a little underbaked in the middle.
Flour: Be sure to fluff and then scoop and sweep to measure the flour. Too much flour will result in a dry or crumbly cookie. Sprinkles: A couple of notes about the sprinkles. I use about 1/3-1/2 cup of sprinkles. This will vary, though, depending on the type of sprinkles that you use. I've had better luck with plain sprinkle mixes. The candies in the sprinkle/candy mixes can melt as they bake. I only coat about the top half of the cookie with sprinkles. Otherwise the sprinkles can melt and make a mess as the cookies bake.
Baking: Don't over-bake the cookies, or you won't end up with soft cookies. Several people have said that they've needed to bake the cookies for longer. Since oven temps can vary, I suggest starting with the listed baking time and increasing the time as needed.