Pop into your food processor the sugar, baking powder and flours and pulse for 3 seconds
Add in the butter, egg yolk, sour cream and vanilla bean paste and blend in the processor for 5-6 seconds or until it just comes together as a cookie dough. For thermomix, do this on speed 5-6 for 5 seconds.
Dust your bench with flour and then tip out your biscuit dough onto the bench. Work it very briefly into a solid mass to catch any little breakaway bits, and then you have two options:
Option 1: Roll out as a flat, 5mm thick pastry sheet and use a cookie cutter to cut cookie shapes out of it for your Ciasteczka - perfect for Christmas or themed birthdays/halloween
Option 2: Roll gently into a log about 4cm thick, ready to cut biscuit rounds out of
Both of the above work optimally if you have time to put the dough in the fridge after either rolling out the sheet and popping onto a chopping board and into the fridge OR after rolling your log, putting the log into the fridge for 30mins before cutting your biscuit disks.
Which ever you choose to do - I'm lazy so I do the log, cut and arrange onto trays.
Then, brush each biscuit with eggwhite and bake in a 180C oven for 12-13 minutes or until golden.
Take out of oven to cool but do not remove from their trays until just warm so that they're stable enough to move - before that they might crumble.
Once cool, dust with a couple of tablespoons of the organic icing sugar through a fine strainer for that lovely snow effect. You can also skip this step but it's a special occasion bikkie and it's pretty so I do it.
Switch oven on to pre heat at 180C/350F