Toast walnuts in a pan for 1 minute and let them cool to enhance their flavor.

In a bowl, soften room-temperature butter and mix it with brown sugar until combined.

Add the egg yolk and stir gently with a whisk until just combined.

Sift rice flour into the bowl and mix with a spatula using a cutting motion.

Chop the toasted walnuts and fold them into the dough until evenly distributed.

Shape the dough into your preferred form (round or square) and wrap it in plastic wrap. Chill in the freezer for at least 1 hour.

Slice the chilled dough into 8mm thick pieces.

Bake in a preheated oven at 180°C for 17–20 minutes until golden brown.