Billie Eilish Shares Her 'Favorite' Cookie Recipe — Made with Costco Peanut Butter!

Billie Eilish gave her fans a sweet surprise on Monday.

The "Happier Than Ever" star posted a video tutorial on her Instagram Story showing followers how to make peanut butter chocolate chip cookies.

Eilish, 19, began by instructing to heat the oven at 350° before adding one cup of brown sugar to a mixing bowl. She then added one cup of peanut butter. Eilish posted a clip of peanut butter dripping out of a measuring cup and into the mixing bowl, writing over the video, "This is so sexual."

The singer noted that she likes to use Costco's Kirkland Signature organic peanut butter, sharing a snap on her Story of the jar and telling her followers, "(fyi this is my FAVORITE PEANUT BUTTER IN THE WORLD so I always use this for everything)."

Returning to her recipe, Eilish shared a clip of the peanut butter and brown sugar swirling together in a bowl, instructing her followers to mix the two ingredients for about a minute. After combining the sugar and peanut butter, Eilish wrote on her next slide, "after one minute of mixing peanut butter and brown sugar, add TWO teaspoons of vanilla extract and mix again for 30 seconds."

Eilish then moved on to the dry ingredient portion of the recipe by showing her followers how to make oat flour. "These cookies are fully vegan and GF and AMAZING so the flour is literally just oats blended up and they work perfectly," she captioned her tutorial.

After blending oats into a powder, Eilish measured out one cup, which she then poured into a separate mixing bowl and combined with a teaspoon of baking soda and ⅛ teaspoon of salt.

"Time to add!!," Eilish wrote on the next slide over a video of herself pouring the dry ingredients into the wet. "I did this very messily but i don't really gaf," she joked.

After the contents of the two bowls were blended into one mixture, Eilish instructed her followers to mix in ¼ cup of almond milk, then finish the batter off with chocolate chips. After stirring the chips in, she scooped out portions of dough and placed them on a baking sheet, sharing a tip before they go in the oven: "squish em down with a fork so they don't spread out," she wrote, adding that the cookies "don't rise too much."

"Get those boys in the oven!!" she wrote over her next slide, which shows Eilish popping the tray in to cook for 10-12 minutes.

Eilish featured the finished dessert in a slide showing the tray of freshly baked cookies, which she captioned, "uhhhhhhhh YUMMMMMM." In a separate photo of the cookies piled into a bowl, she wrote, "they're even 1,000 times better than they look," adding, "enjoy!!!"

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Eilish ended her cooking lesson on a funny note. The singer posted a photo of the bowl of cookies looking noticeably less full than in the previous image, writing, "I left the room for two second[s].." and sharing a snap of her very guilty-looking dog on the next slide with the word "bad" written below the pet.

The singer is sharing her classic cookie recipe not long after she launched her first food product, a vegan chocolate bar. Eilish, who has been vegan since she was 12, promoted her "Happier Than Ever" bar in her Instagram Story Monday, telling her followers, "before I stopped eating dairy, milk chocolate used to be my favorite thing on earth and since i've gone vegan i could never find a good vegan milk chocolate, and never really loved dark chocolate."

Now that she's created her own chocolate, Eilish is using it to elevate her go-to dessert recipe.

"I like to top these cookies off with a piece of my milk chocolate," she told her followers.