McDonald's is serving up McFlurrys from a "walk-thru" billboard on London's South Bank.
McFlurry fans who pass by the Builder's Yard under Hungerford Bridge can collect the free treat from a hatch on the interactive sign that is open between 11am and 6:30pm until 22 August.
Through the built-in service window, consumers will receive a McFlurry made with vanilla ice cream and Oreo toppings.
Leo Burnett London worked in partnership with OMD and Talon to deliver the activation, which is also part of the ongoing "Fancy a McDonald's?" brand platform.
Steven Howells, director of marketing and media, McDonald's, said: "We all need moments of light-hearted release – opportunities to let go and forget the pressures and expectations of life. McDonald's, in its own way, has been providing small moments of joy to the UK for over 40 years. And as McFlurry celebrates 21 years on our menu, it seems only right to surprise and delight our customers by giving some away in a truly unique way."
Mark Elwood, executive creative director at Leo Burnett, said: "McDonald's has always been a generous brand and our 'Fancy a McDonald's?' brand platform allows that thinking to come to life in less traditional, more unexpected ways. Fancy a free McFlurry, anyone?"
The "Fancy a McDonald's?" brand platform was launched on 2 July with ad "Laughter" by Leo Burnett, which features no dialogue, just giggles and chuckles. McDonald's went on to release "Me time", which centred around the personal delight a McDonald's can bring.
As part of McFlurry 21st birthday celebrations, a McFlurry van has been visiting at-home birthday parties.