Melt butter and let it cool slightly (just warm).
Mix in white + brown sugar until smooth.
Add in the egg yolk and vanilla, mix well.
Stir in the flour, baking soda, salt.
Preheat oven to 170°C (up/down).
Scoop into balls (50g each), roll in sugar if you like.
Chill them for 10 - 15 min in the fridge
Bake for 9–11 minutes until edges are set and centers look puffy but soft.
Let cool for 5–10 mins on tray to set fully.
