Preheat your oven to 170°C (338°F). Line your baking tray(s) with baking paper.
Sift the flour and baking powder together.
Beat the sugars, peanut butter and softened butter together until well mixed, creamy and smooth-ish (no lumps).
Add the egg and beat until well combined. Scrap down the sides of the mixing bowl so that everything is well mixed.
On a low speed stir through or gently fold in the flour and baking powder until mixed through - but don't over mix it.
The dough will be rather soft so I prefer to refrigerate it (about an hour) until its cold and stiff enough to handle with my fingers and roll in my palms - but try to work quickly even if you're working with refrigerated dough. You can choose not to refrigerate if you don't want to or cannot wait.
Roll heaped teaspoonfuls of the dough into balls and place onto the baking tray.
Then use the back of a fork (lightly dusted with flour every now and again so it won't stick) and flatten the balls of cookie dough down a little into discs.
Bake between 10-12 minutes, rotating your tray at half time, until the cookies are lightly brown. Your cookies should be just that bit soft if you gently poke with at it.
Do this quickly - before the cookies cool down and while it's still hot and warm from the oven, sprinkle some salt over the cookies.
Then cool the cookies on the tray - the cookies will crisp up on the outside and remain just that bit chewy inside.
The dough yielded around 40 cookies for me but it'll all depend on how big/small you make these cookies.