Preheat the oven to 160C / 325F. Line a loaf pan (roughly 9 x 5 inches) with baking paper and set aside.
Leave the butter out of the fridge for a while to soften, then add it to a large bowl with the sugar and cream it together with a hand-held whisk until light and fluffy (two or three minutes).
Add in the eggs one by one and whisk until well combined, then whisk in the vanilla.
Tip in the flour and baking powder and stir in with a wooden spoon until well mixed (don’t be tempted to over-mix).
Pour roughly 1/3 of the cake batter into the bottom of the loaf pan. Then warm the Nutella in a microwave* (15 seconds or so should do it) and pour half of it over the batter in the pan.**
Pour another 1/3 of the batter over the Nutella layer, followed by the rest of the Nutella. Finally, top with the last layer of cake batter.
Sprinkle with hazelnuts, pressing lightly into the top, then bake the cake for about fifty minutes or until the cake is well risen and has begun to pull away from the sides of the pan.
Let cool for ten minutes in the pan, then remove and let cool on a wire rack before slicing.
Warming the Nutella
If you don’t have a microwave, just warm the Nutella it slightly over a pot of simmering water.
How to layer the cake batter
Try to leave a small border between the layer of Nutella and the sides of the pan. This will stop it running out of the cake too much.Don’t worry if you don’t get it exactly right. It won’t really matter.