Place flour and cubed butter into a bowl of vitamix in listed order and pulse on speed 5 a few times until mixture resembles crumbs (3-5 pulses within 3-4 second intervals).
Move the mixture to a bench or a mixing bowl, make a well in the centre and add sour cream and egg yolk. Mix gently until dough comes together. Divide dough into 2 equal pieces, shape into discs, wrap in cling film and set aside to rest for at least 30 minutes.
In the meantime make the walnut filling, prepare and line cooking tray and preheat oven to 160C Fan Forced.
To make the walnut filling place all ingredients into the bowl of vitamix and pulse on speed 5 until your mixture turns into a paste with some walnuts still remaining in small chunks for texture.
Place one of the dough discs onto lightly floured bench and roll to 5mm thick. Cut into 8 triangles like you would with pizza. Place 1 tsp of walnut mixture onto wide end of each triangle, tuck the wide sides and roll towards the end point.
Place mutekke cookies on a tray and bake for 20-25 minutes or until golden. Remove from oven, move to a cooling rack. Once cooled roll in icing sugar.