Which Roald Dahl novel features the magic potion 'Formula 86 delayed-action mouse-maker'?

The Witches is a children's fantasy novel by the British writer Roald Dahl. It was published in 1983 by Jonathan Cape in London, with illustrations by Quentin Blake. The story is set partly in Norway and the United Kingdom, featuring the experiences of a young boy and his Norwegian grandmother in a world where child-hating evil witches secretly exist.

In chapter 15 the Grand High Witch scolds the English witches for their lack of progress in eliminating the children, incinerating one of them when she talked back about her demands for the death of every English child within a year. The Grand High Witch unveils a master plan. All of England's witches are to purchase sweet shops (with "homemade" money printed from her money-making machine). They will then give away free sweets and chocolates for the grand opening laced with her latest creation, "Formula 86 delayed-action mouse-maker", a magic potion meant to turn the drinker into a mouse at a specific time with a single drop. The intent is that the children's teachers can kill the transformed children for them.