Beat the butter, chocolate-hazelnut spread, caster sugar and brown sugar with an electric mixer in a large bowl until smooth. Blend the whole egg into the butter mixture. Beat in the egg yolk and vanilla.
In a separate bowl, sift together the flour, bicarbonate of soda and salt. Mix the dry mixture into the egg and sugar mixture about 1/3 at a time until all the flour is incorporated. Gather the dough into a ball and wrap in cling film. Refrigerate at least 30 minutes.
Preheat an oven to 200 C / Gas 6.
Roll pieces of cookie dough into balls (about the size of a golf ball) and place 5cm apart on ungreased baking tray. Bake until cookies are slightly firm on the outside, 8 to 10 minutes. Cool cookies on trays before removing.