Preheat the oven to 180°C and grease and line a loaf tin with baking paper. You can also bake these in a 20cm square tin for slightly thinner brownies, although you may need to double up on the shortbread base.

For the shortbread base, cream the sugar and butter with an electric mixer until pale and creamy. Mix in the flour to form a stiff, smooth dough. Crumble the dough into the base of your prepared tin and press into an even layer.

Bake for 10-15 minutes or until the edges start to turn golden. Remove from the oven and set aside to cool slightly.

For the brownie layer, mix together the sugar (sachet 1) and melted butter, then mix in the eggs until well combined. Add the brownie mix (sachet 2) and stir to form a smooth, glossy batter. Pour on top of the cooled shortbread base and smooth the surface.

Heat the Nutella in the microwave in 10-second bursts until softened, but not runny. Place into a piping bag or sandwich bag (see Tips) and snip a small hole off the end. Pipe lines of Nutella widthwise across the brownie batter. Drag a butter knife through the Nutella lengthways, alternating directions each time to create a feathered pattern.

Bake for about 25 minutes or until cooked through but still fudgy in the centre. A slightly raised, puffed center is a good sign that the brownie is cooked, but be careful not to overdo it as this will result in a cakey texture. Bang the tin on your work surface 2 or 3 times to deflate the brownie and flatten the surface.

Allow to cool completely before slicing into squares or bars, and sprinkle with some sea salt flakes for an interesting finish.

Tip: If you’re not invested in the feathered pattern on top, you can simply dollop the Nutella on the surface of the brownie and swirl in with a knife.