Preheat the oven to 180⁰C – Gas 4. Butter and line the base of a 20cm round cake tin. Put the butter, sugar, eggs, flour, baking powder, cinnamon and milk into a bowl. Beat until light and fluffy.
Tip three quarters of the mixture into the tin, spread it level, then spoon the hazelnut spread on in 4 blobs. Top with the remaining mixture, swirl a few times with a skewer, then smooth. Sprinkle with the nuts.
Bake for 1 hr – 1 hr 10 min, until risen, nicely browned, feels firm to the touch and springs back when lightly pressed (cover with foil if it starts to brown too quickly). Cool in the tray for 10 min, then turn out, peel off the paper and cool on a wire rack.