Place an oven rack just below the centre of the oven and turn it to 160°C fan/convection oven or 180°C conventional oven.
Line a 27 x 16 cm standard-sized, slice tin with non-stick baking paper.
If you only have walnut halves, you'll need to roughly chop them up into pieces and then set them aside.
Place the walnuts in an ovenproof dish and toast them in the oven for 5 minutes, toss them well and then return them for another 3 - 5 minutes or until they are 2 or 3 shades darker. Then leave them aside to cool down.
Turn the oven down a little, to 150°C fan/convection oven or 170°C conventional.
If your chocolate is in a block use a large knife to roughly chop it up.
Place the butter and vanilla in a small saucepan and over a low heat, melt the two together.
Whilst the butter’s melting, place the sugar and cocoa powder in a medium-sized bowl and mix the two together.
Pour in the still warm butter and stir it in well.
Next, add the eggs, one at a time mixing well after each addition.
Add the toasted walnut pieces, chopped chocolate (or buttons) and the flour, and fold it all together until just combined.
Give the lined tin a light going over with cooking spray and then spoon in the brownie batter, spreading it around evenly.
Place it in the oven and it should be cooked in 40 minutes, but check by inserting a skewer or a little sharp knife into the centre and if it comes out clean it’s ready.
Once cooked, leave it in the tin for 10 minutes, to cool down a little.
Tilt the tin and use the paper to carefully lift/slide the brownie (paper too) out onto a cutting board to allow it to cool down to room temperature before slicing it.
Store in the pantry in an airtight container, with some non-stick baking paper between the layers for 3-4 days.