Preheat the oven to 180oc/160oc fan and grease and line a square 20cm tin with baking parchment. Place the butter in the bowl of a stand mixer or a large bowl if using a handheld mixer, and beat until pale and creamy for around 1 minute. Add the sugar and beat again until the two combine to a pale brown colour. With the mixer running, pour in the eggs and vanilla paste and mix until well combined, scraping down the edges of the bowl with a spatula. Add the Nutella and salt and beat again until well combined. Add the flour and carefully fold into the mix, taking care not to overbeat. Stir through the chocolate chips.
Pour the brownie batter into the tin and level with a spatula. Place 9 teaspoons of Nutella across the batter and using a skewer, marble together with a swirling motion. Sprinkle over a little more salt then bake in the oven for 30 to 35 minutes until a skewer inserted into the middle comes out with a few moist crumbs, taking care not to stick the skewer into the melted Nutella. Once baked, remove from the oven and place the tin on a wire rack. Let the brownies cool completely in the tin before lifting out and cutting into squares or decorating as a birthday surprise. Brownies will keep in an airtight container for around 5 days.
Recipe by Victoria Sponge Pease Pudding at http://victoriaspongepeasepudding.com/baking/smoked-salt-and-nutella-birthday-brownies/