The X-Men star took on Dame Kelly Holmes, David Baddiel and Anne-Marie in this week's competition in aid of Stand Up To Cancer, but despite claiming a lack of experience, he managed to ace every challenge.
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First up, McAvoy made a raspberry version of a lemon meringue pie which he had invented himself.
An impressed Prue Leith told him: "I would have thought you couldn't improve on a lemon meringue pie, but this is better."
Fellow judge Paul Hollywood said: "You're so close to perfection with that."
Dame Kelly also did well with her tart, as Hollywood told her: "That pastry is one of the best I've had for a while."
Next, McAvoy triumphed in the technical challenge by winning the task to create perfect cheese scones and butter.
Leith commented: "They are pretty much a perfect scone, you pay a lot of money for stuff like that."
However, Anne-Marie, despite coming second, started the task by saying: "What the hell is a cheese scone?"
The showstopper task was to make a 3D marble cake which had to represent the celebrity baker in the form of an animal.
McAvoy chose to create a snow leopard in tribute to his character Lord Asriel in the TV adaptation of His Dark Materials, and came up with a bake that wouldn't have looked out of place in the main series.
Hollywood told him: "For someone who doesn't bake, that's surprisingly good."
Of course, McAvoy was crowned Star Baker, but had competition for the title from both Dame Kelly and Anne-Marie.
Baddiel struggled the most, with a series of melted tarts, collapsed scones, and a recreation of his cat Chairman Miaow which didn't quite go to plan.
Celebrity Great British Bake Off continues next Tuesday night on Channel 4.
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