Mini Moni the Movie: Okashi na Daibouken!

It's almost the second anniversary of Mini Moni's founding, and everyone at the Mini Moni Cafe is busy preparing a fabulous cake shaped like a castle to celebrate. Cafe leader Mari Yaguchi oversees baker Mika Todd and waitresses Nozomi Tsuji and Ai Kago, but when the curious customer Ai Takahashi wants to know why the cakes they make taste so great, they're unable to answer. She decides to find out for herself by stealing the cakes, but soon deeper trouble forms.

(Source: TV Tropes)