Preheat oven to 160 C / Gas 2/3. Grease a baking tray.
Beat together the butter and sugar until it is pale and fluffy then add the vanilla extract and the flour and mix until a soft dough forms.
Divide into two equal pieces and work the Nutella® and the cocoa powder into one. On a floured surface, roll each dough out equally (If it is too soft then wrap it up in cling film and put it into the fridge for about 10 minutes).
Gently lay the chocolate dough over the other and trim the edges so they're even. Roll it up lengthways and cut into about 6mm slices it should reveal a swirl.
Bake in the preheated oven for 10 to 12 minutes until firm and golden brown. Remove from the oven and allow to cool before enjoying.