Bring the butter to room temperature, and cream with a whisk.

Add sugar and mix.

Add the egg yolk, and mix until the mixture becomes smooth.

Finally, add cake flour, and fold in.

Roll into a ball, cover with plastic wrap, and let it sit in the fridge for about 1 hour.

Roll out the dough to 5 mm thickness, and cut out shapes. Bake in the oven for about 20 minutes at 170℃/340℉.

I usually let them cool on newspaper. It absorbs the oil well. Let cool completely, and they're done.