Lonely?

Both you and your partner each need a Buttermilk Delight in your inventory before starting — only the player who wants the achievement is strictly required to have one, but if your partner also needs it, they must bring their own.

Go to Northrend Dalaran (the Wrath of the Lich King city) and place the Romantic Picnic Basket in a general, unnamed area of the city — avoid named sub-districts, as the achievement may fail there.

Click the Romantic Picnic Basket to sit down — do not use /sit, as only clicking the basket grants the required buff. Your partner must also click the basket to sit alongside you. Note that you do not need to own the Romantic Picnic toy yourself; any player who has it in their toy box can place the basket.

After both players are seated, wait a short moment until a heart appears above your head and you hear a distinctive tune, confirming you have the Romantic Picnic buff active.

Once both players are seated and the Romantic Picnic buff is active, eat your Buttermilk Delight to complete the achievement — eating it before the buff appears will not count, and you will need another Buttermilk Delight if you do so prematurely.