Before you do anything at all, pop the white chocolate into the freezer while you get everything else ready. It’ll help the pieces hold together during baking, instead of melting into the brownies and disappearing.
Preheat your oven to 340⁰F/170⁰C, combination steam setting. If your oven has variable steam settings, use 50-60% steam. If not, don't worry! Just set to combi steam at the correct temperature and the oven will figure out the humidity. Line a 20cm/8in square pan with baking paper and extend it up the sides for ease of removing the brownies later (I use a cake pan but a shallower tray would be ideal).
In a saucepan over medium heat, melt the butter and sugars together. You want the sugar to mostly dissolve otherwise your brownies will be grainy. Remove the pan from heat and immediately drop all the dark chocolate in. Give it a stir and leave to sit for a minute or two while the residual heat melts the chocolate (beware: if you stir too much the mixture can split and it won’t ever really come back together, so just mix it a couple of times until it’s combined and smooth).
Once the chocolate has melted completely and the mixture is cool enough to touch, mix in the eggs and vanilla. Fold through the flour, then the pecans and cold white chocolate.
Spread the mixture into your pan and bake for 15-16 minutes, or until a skewer tests almost clean (a few crumbs on the skewer are ok). Cool to room temperature, then chill until completely cold before removing the whole slab from the tin and cutting into bars or squares.
These will keep in a sealed container in the fridge for about 5 days, if you can make them last that long.