Preheat oven to 400°F. Line 3 baking sheets with parchment paper.
Add sugar and butter together in a large bowl and beat with a hand-held mixer until light and creamy, about 2 minutes.
Add baking powder, extract, and salt, and mix until combined.
Add eggs, one at a time, until completely combined. Scrape the sides of the bowl as needed.
Alternate adding the flour and milk to the butter mixture and mix until completely combined. (Do not overmix)
Add rainbow jimmies to the batter and gently combine by hand.
Using a large cookie scoop, scoop out batter and place on cookies sheets about 2 inches apart. Bake for 8-10 minutes, or until edges are slightly golden brown.
Remove from oven and let them cool completely before adding frosting.