All my friends agree that I’m a good cooker!
This is a good try, but you are over-using the rule that says you can change a verb into a profession by adding ‘er’:
As with so many rules in English, there are exceptions. If you cook then you are a cook!
A cook (or ‘chef’)
A kitchen ‘cooker’
If you really are – as your friends say – a cooker, then you are a domestic appliance used in many residential homes for heating food.
Good luck with that! 🙂