350* oven.
Line your baking sheet with parchment paper. (Sarah says to use foil, dull side up, but I prefer biodegradable parchment paper).
In a small bowl, combine the dry ingredients: the flour, baking soda, salt, and the cream of tartar.
In the bowl of an electric stand mixer, cream the unsalted butter for one minute or until creamy.
Add in the 1 1/2 cups of sugar and beat on medium speed until the mixture looks light and fluffy for 2-3 minutes. Stop the mixer and scrape down the sides with a scraper as necessary. Add the large egg and the vanilla extract and set the speed to low on your stand mixer. Combine.
Add the flour mixture and combine on low speed. When you see a couple of streaks of flour remaining, add in 1/2 cup of "jimmies" (the longer sprinkles, not the short shiny sprinkles because those will disintegrate) and the semisweet chocolate chips (if using), and mix until just combined. Don't overmix your funfetti cookie dough. Seriously, don't overmix.
Using a 1/4 cup cookie scoop, dish out a dough ball and roll it in the remaining 1/4 cup of granulated sugar and more Jimmie sprinkles (the longer colorful sprinkles).
Set each dough ball scoop on the prepared pan, four per pan if you are using the 1/4 cup cookie scoop for your funfetti sugar cookie dough. If you are using a smaller 1 1/2 Tablespoon cookie dough ball scoop, then you'll want to leave a 1 1/2" to 2" space between the sprinkle sugar cookies so they have room to expand. EIGHT minutes into baking, when your cookie dough balls look a little puffy in the middle, lift one edge of your cookie sheet pan up and let it drop back down onto the oven rack. This will feel wrong. Ignore the feeling. The cookie dough ball edge will set, and the center will flatten. After 2 minutes, you'll see the center of the funfetti cookies puff again. Repeat the pan banging. Keep repeating the process at least twice until you've reached a 12-15 minute bake time and your cookies look done (the edges of the funfetti sugar cookie will appear golden brown, you'll smell the scent of baked cookies, and the centers will be lighter but not completely cooked. If you wait to remove the cookies once the center is thoroughly baked, you will overbake the cookies, and they will be too dark and hard. After you bake cookies so the center is almost set, move the hot baking sheet to a cooling rack away from the oven vent or other heat source. Leave the cookies on the cookie sheet on the rack for 10 minutes.
After 10 minutes, transfer the sprinkle sugar cookies off the cookie sheet to the cooling rack to finish cooling.
Store cookies in a plastic airtight container, so they don't get hard (don't use a cookie tin!). Sarah says you can pop them in the fridge for three days. Leaving them at room temperature will net you two days before the texture might change.