As business magnate Warren Buffett turned 90, his close friend and billionaire philanthropist Bill Gates baked a special cake for him.

In a video that has gone viral on social media, Bill Gates can be seen baking a special Oreo cake bearing Buffett’s face.

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“Warren has the mental sharpness of a 30-year-old, the mischievous laugh of a 10-year-old, and the diet of a 6-year-old,” Gates wrote in his blog.

“Here’s a short birthday video in honour of his dietary preferences,” he added, sharing the video.

In the clip, one can see Gates entering a kitchen ready to bake a cake.

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The remaining video shows a close-up of the entire baking process followed by the big reveal – the finished cake with Buffett’s face on it.

At the end of the video, Gates cuts a piece of the cake and puts it on a plate. The message “Happy 90th Birthday Warren” can be seen written next to it.

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Happy 90th birthday, Warren!

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In the blog, Gates mentioned his and Buffett’s love for math and numbers and shared some numbers about turning 90 and their friendship.

“10,649: Days since we met for the first time, on July 5, 1991,” Gates wrote.

“2: Phone numbers I have on speed dial at my office- Melinda’s and Warren’s,” he wrote further among other points.

The Microsoft co-founder also shared that the most important thing he has learnt from Buffett is “what friendship is all about”.