I found this amazing recipe on http://www.food.com
I made a little change according to it:
Cocoa powder 30g
Sugar 150g ( I already reduced sugar based on original recipe but still found it is very sweet.)
Procedure is almost same as original recipe:
1. Melt butter, add flour, cocoa powder and sugar.
2. Add in eggs. Stir well.
3. Pour into pie plate.
4. Preheat oven to 190 degree C, bake for around 30 mins. (After 30 mins, the edge is a little burnt, so I think maybe 25 mins is also fine.)