"Euphoria, I take it? And what's that I smell? Mmmm . . . you've added just a sprig of peppermint, haven't you? Unorthodox, but what a stroke of inspiration, Harry..."
-- Horace Slughorn (HBP22)
The Elixer to Induce Euphoria is a potion which induces euphoria in the drinker, with occasional side effects of excessive singing and nose-tweaking (the addition of peppermint would tend to counterbalance that effect) (HBP22). When Harry was assigned the task of concocting this potion by Slughorn, he found that it was improved by following the notations by the Half-Blood Prince in Harry’s copy of the textbook (HBP22).
References from the canon
When correctly made, the potion is sunshine-yellow (appropriate, being a colour suggestive of cheerfulness) (HBP22)
Instructions to brew this potion can be found in Advanced Potion-Making (HBP22)