cream butter with sweeteners and vanilla extract until light and pale in colour.
incorporate egg yolk, then flours.
without overworking the dough, shape it into a ball, flatten and wrap in cling film; refrigerate for 1 hour.
once chilled, unwrap the dough, place it over a silicone pastry mat and place cling film over it.
roll it out to 5mm and cut desired shapes, reworking the dough until you've used it all up.
place the biscuits on an oven tray lined with non-stick baking paper and chill for 20 minutes, then bake in pre-heated oven at 160°C fan for about 10 minutes, until they start browning around the edges.
allow the biscuits to cool thoroughly before handling.
Notes
The best way to ensure accurate measurement of ingredients is with Metric Kitchen Scales. They are inexpensive and take up very little space. Click HERE for the ones I use. For U.S. option click HERE.