Take the butter out of the fridge at least 30 minutes before starting and pre-cut it into small cubes. The butter should be very soft but not melted.
Separate the egg yolks from the whites. Keep the egg whites in the fridge for another recipe.
In a large mixing bowl, whisk together the egg yolks and icing sugar until smooth and pale.
Add the very soft salted butter and mix until you get a thick, soft mixture.
Sift in the flour and baking powder. Combine using a stiff spatula or hands until the dough is smooth.
Place the dough between two sheets of baking paper and roll to about 6 mm thick. Chill in the fridge for at least 1 hour.
Preheat your oven to 160°C (325°F). Line a baking tray with parchment paper.
Remove the baking paper and cut out cookies with a round or fluted cutter. Transfer to the baking tray.
Optionally, create a cross pattern with a fork and brush each cookie with an egg yolk for a golden finish.
Bake for 15-18 minutes until golden. Cool on a rack and store in an air-tight container for up to 5 days.