Preheat oven to 160°C. Line a baking tray with baking paper.
Combine peanut butter, caster sugar, brown sugar and eggs in a large bowl. Stir through chocolate melts, reserving a handful for topping the cookie. Use your hands to shape the cookie dough into a ball, then flatten on prepared tray to about 3cm thick. Press in reserved chocolate melts.
Place tray in oven and bake for 30 minutes. Remove from oven and allow cookie to cool slightly to firm up, then serve warm.