Preheat your oven to 185°C (or 365°F) if using a fan oven or 200°C (395°F) if using conventional oven.
Place sliced butter into a light-coloured, heavy bottomed pan or skillet. Melt over medium-high heat, stirring continuously, until it becomes foamy, then turns clear and starts showing brown specks. You'll also notice nutty aroma. It's done after approx. 5 min, at medium brown stage. Transfer the browned butter to a medium sized bowl and allow to cool a little bit.
While the butter is cooling, prepare the rest. In a medium sized bowl whisk together the flour, baking soda, baking powder and salt.
Using a sharp knife, chop good quality dark chocolate to uneven chunks (mix of small and slightly bigger ones). 250 g of chocolate goes directly to the cookie dough, while the other 50 g is saved to put on top of the cookie dough balls, prior to baking. To get that chocolate pools on top. :)
Once the brown butter has cooled and it's not hot to the touch, add the sugar, vanilla extract and eggs and mix it using a whisk until it's incorporated, smooth and slightly paler in colour. There 's no need to beat a lot of air to it.
Add the dry ingredients to the butter mixture and combine them using a wooden spoon or a rubber spatula, until combined and there are no traces of flour. A this point, the dough will be sticky and soft.
Next, add in rolled oats and chocolate chunks and combine with a wooden spoon, or better yet - using your hands. If you don't wish to get dirty, use latex gloves.
Cover the dough with some plastic wrap or aluminium foil and leave to chill in the fridge at least two hours. A day would be ideal, because the flavours really deepen and come through nicely. But a couple of hours in the fridge will do the trick for a chewy cookie that's soft on the inside and crispy on the outside.
(If you're into thin and crispy cookies, then form the cookie dough balls and bake straight away.)
Using your hands form 70-75g cookie dough balls and place them on a cookie sheet lined with baking paper leaving 2 inches (5 cm) between each cookie dough ball. Top each cookie dough ball with a couple of chocolate chunks.
Bake the cookies for 11-12 minutes. They will be soft in the middle when they're done, with golden brown, crispy edges. Leave them on the tray for a minute, to firm up a little bit, and then transfer them on a cooling rack to cool completely.
TIP: To get those pools on top, smush the visible chocolate chunks with a back of a teaspoon once the cookies are done and have had a minute to firm up. :)
Keep them in an airtight container to prolong the freshness.