These great looking cookies are deceptively easy to create... vanilla and chocolate cleverly interlaced.
Beat the softened butter and sugar for the vanilla cookies together until pale and fluffy. Crack in the egg yolks and vanilla extract, beating together. Fold in the flour until fully combined.
Take just over half of the cookie dough out of the bowl and leave to one side. Beat the 25g of butter, milk and cocoa powder into what's left in the mixing bowl. Combine to make sure it's all evenly chocolate-y... sometimes this is easier by hand than with a spoon!
Roll both the vanilla and chocolate cookie dough out between separate sheets of baking paper to the thickness of about 1/2cm and the size of roughly an A4 sheet. Lay the chocolate sheet on top of the vanilla one, peel back the paper and them roll the two dough together like a Swiss roll, keeping it tight, making sure no paper is trapped inside. Chill in the fridge, wrapped in clingfilm, for an hour or so to set up.
Preheat an oven to 170ºC. Slice the cookies into discs 1cm thick, then lay on a lined baking tray, keeping some space between them, for 15 minutes.