Preheat oven to 350 degrees F.
In a large bowl, Using a hand or stand mixer, cream the butter on medium speed for about 20 seconds.
Add the sugar and mix until well combined. (about 30 more seconds)
Add the egg and vanilla. Continue to mix on medium speed.
In a separate bowl, whisk together the flour, salt, baking powder, baking soda, and cream of tarter.
Add the dry ingredients to the wet ingredients in three different parts. Mixing together on low speed.
Once well combined, use a rubber spatula to fold in the sprinkles, marshmallows, and white chocolate chips.
Line a cookie sheet with parchment paper, a silicone baking mat, or spray with non stick spray.
Scoop about 3 tablespoons of dough, and roll into balls.
It is optional to roll the outsides in sprinkles again. ( You can never have enough sprinkles) (;
Place the dough balls on the cookie sheet and bake for 10 minutes.
The cookies will appear quite soft, but allow them to cool on the sheet for about 5 minutes, and then transfer them to a cooling rack to completely cool.