Preheat oven to 350°. Prepare a parchment paper-lined baking sheet and set aside.

In a small glass bowl, microwave unsalted butter (¾ cup, i.e. 1 ½ sticks) until melted. Pour into large mixing bowl. Add in granulated sugar (½ cup) and powdered sugar (½ cup) and stir until incoporated.

Add whole egg (1 large), extra egg yolk, vanilla extract (2 teaspoons), and cake batter extract (½ teaspoon) and stir until incorporated.

Add baking powder (1½ teaspoons), baking soda (½ teaspoon) and salt (¼ teaspoon). Mix until combined. Half cup at a time, add all-purpose flour (2½ cups) and mix until flour is just incorporated. Mix in rainbow sprinkles (¼ cup).The texture of the dough should be similar to playdoh (it will firm up after a few minutes as the melted butter cools). If it looks runnier than the photo above, add 1-2 tablespoons of flour.

Scoop dough into ¼ cup (i.e. 4-tablespoon sized) scoops, then roll into a ball. Lightly flatten each ball before placing on cookie sheet. (Because this has melted butter, do not let the dough sit out before rolling into balls & baking, or your dough will become dry and crumbly.)

Bake at 350° degrees for 11-12 minutes until center no longer looks wet and the edges have the tiniest amount of brown. They may look slightly underdone, but they will finish baking on the hot cookie sheet after you take it out of the oven. (Of course, because ovens bake differently use your best judgment to keep them in a minute longer if they look severely underbaked.)

After removing from oven, allow the cookies to cool on the hot baking sheet for 10 minutes before removing them from cookie sheet.

Note: if you like a THICK layer of frosting, double the frosting recipe!

In a large bowl, beat the unsalted butter (¼ cup, i.e. ½ stick) and cream cheese (4-ounces) on medium speed using a hand or stand mixer (with paddle attachment) for about 1 minute (until light and creamy).

Keeping the mixer on medium speed, slowly add in vanilla extract (1 teaspoon), cake batter extract (¼ teaspoon) and powdered sugar (1½ cups). Once incorporated, turn mixer to high speed and beat for 2 minutes until creamy.If your frosting is too thick, add 1-2 tablespoons of milk to thin it out. If it it too thin, add a bit more powdered sugar.

Spread or pipe frosting on cooled cookies and top with sprinkles! For the perfect cookie swirl, I use a large round tip. Piping tip! When piping, hold the tip very close (but not touching) the cookie to get a even layer of frosting. Holding it too high above the cookie & swirling it will result in a THICK layer and you will likely run out of frosting before you frost all the cookies ;) If you like a very thick layer, 1.5x or 2x the frosting recipe.

Store the frosted cookies in an airtight container in the refrigerator. These cookies stay soft & moist for 3-4 days.