What's the best diet you've ever had for losing weight and muscles?

So basically the thing is, I want to lose 8 kilos. But because I used to do running for years when I was younger, my body builts up muscles extremely fast. Like if I cut down my food a little bit and start exercising even the slightest, my body will just built up muscles. Something I don't want. I want to get at 50 kilos (I'm now 58kilos, and height 1,64) and once there I will start to exercise to tone my body when it's going to be thin.
So do you have anything to suggest? What should I do? What is the best diet you've ever tried?
What's the best diet you've ever had for losing weight and muscles?
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