Preheat oven to 180 degrees F. Cover two baking trays with baking paper.
Beat the sugar and butter together until creamy. Add the egg yolks, vanilla bean seeds and mix. Sift the flour and baking powder together and beat in.
With your hands, form small balls and place 5cm apart on the baking trays. Bake 10 minutes, until the edges are just starting to brown. Let cool on a rack.
Spread half the cookies with Nutella and sandwich with the other half.