Starbucks' PSL makers poured espresso shots over pumpkin pies to perfect the flavor.

For flavor testing, Dukes and his Starbucks team poured espresso shots on pumpkin pies.

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Dukes believes the PSL recipe won't change for almost a decade.

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Starbucks created their Pumpkin Spice Latte taste. How?

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Pumpkin pie and espresso shots galore.

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Starbucks is commemorating the fall drink's 20th anniversary, so Insider spoke with Peter Dukes, the "father of the PSL," about its origins.

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Dukes told Insider that he and his crew brainstormed 100 fall drink ideas in Starbucks'

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"Liquid Lab" in Seattle in early 2003. One concept was a pumpkin latte.

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