Beat butter, icing sugar, egg yolk and vanilla extract until just combined (about 30 seconds).
Mix in plain flour, ground cashew nut, custard powder and baking powder to form a pliable dough.
Place dough in between 2 plastic sheets, roll into 5mm thick. Cut into your preferred shape using a cookie cutter.
Place onto a baking tray lined with baking paper. Press half or whole cashew nut on top of the cookies and glaze with beaten egg yolk.
Bake in a preheated oven at 170 deg C for about 20-30 minutes or until golden brown.
Cool cookies on cooling rack before storing them into an airtight container.