Preheat oven to 180°C/350°F (160°C fan). Lightly grease then line three baking trays with paper.

Brown butter - Melt butter in a small silver pan or saucepan over medium heat (Note 2). Leave to simmer on medium heat for 3 to 5 minutes, stirring every now and then, until (when you push foam aside) you see little golden-brown bits. Immediately pour the butter and all the brown bits into a large mixing bowl (large enough to mix the cookie dough) and set aside to cool for 5 minutes.

Chop choc - Cut the chocolate into 1cm /1/4" pieces. Transfer the chunks into a bowl and leave the fine chocolate dust behind for another purpose (makes cookies speckled, Note 1).

Mix dry ingredients - Whisk flour, oats, baking soda, and salt in a large bowl.

Back to the butter - Add sugar to the brown butter and whisk (it might not combine fully). Add egg and vanilla, whisk until smooth.

Finish dough - Add dry ingredients into the egg-butter mixture. Mix with a wooden spoon until the flour is almost mixed in. Add chocolate, and stir until mixed through and you can no longer see flour. Set aside for 5 minutes to let the mixture firm up a bit.

Cookies - Scoop a heaped tablespoon of dough onto the tray, 5 cm / 2 " apart. (I use a size 40 cookie scoop, makes 22 cookies) Flatten to 8 mm / 1/3" thick, reshaping sides if needed if they split/go wonky.

Bake - Bake 2 trays for 11 minutes, switching shelves and rotating the tray at the 7 minute mark, or until the surface is light golden. For 1 tray, bake 10 minutes, rotating the tray at the 6 minute mark.

Cool on the tray for 10 minutes, then transfer onto a cooling rack to fully cool. (A sprinkle of salt flakes wouldn't go astray!)