Microwave the butter for about 40 seconds. Butter should be completely melted but shouldn't be hot.
½ cup butter
In a large bowl, mix butter with the sugars until well-combined.
½ cup granulated sugar, ¼ cup brown sugar
Stir in vanilla and egg until incorporated.
2 teaspoons vanilla extract, 1 large egg
Add the flour, baking soda, and salt. Please read the recipe note about properly measuring flour.
1 ¾ cups all-purpose flour, ½ teaspoon baking soda, ¼ teaspoon salt
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.
Sprinkles
Bake for 7-10 minutes, or until cookies are set. They will be puffy and still look a little underbaked in the middle.
Notes
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.