Preheat oven to 350F/180C. Line a small baking dish with parchment (around 7x5 in/ 18x13cm).
Place the butter and chocolate in a small saucepan and warm over a medium-low heat until both have melted. Stir now and then to ensure they don't burn and remove from heat once just melted.
Meanwhile, sieve the almond flour, chestnut flour and cocoa powder into a bowl. Add the salt and mix all together well (either spoon or with whisk) so well combined and without clumps.
Add the white and brown sugars to the melted butter-chocolate mixture, making sure they are well mixed in. Then add the eggs, one at a time, mixing fast and well so that they are well combined but you also don't get any bits cooking in the heat of the pan.
Add the vanilla extract and mix in, then add the flours mixture. Stir it in so well combined, and when near being done, press on any lumps of flour to help them burst and mix in.
Pour the mixture into the lined baking dish, spread the mixture out to the corners then flatten the top slightly. Bake in the preheated oven for approximately 30 -35 minutes until a skewer tested in the middle comes out clean. It will puff up a little then fall back as it cools.
Leave to cool in the dish for around 10 minutes or more before loosening the edges and removing the block of brownies from the dish with the parchment. Cut into pieces and enjoy warm or let them cool further.
Video
Notes
You can easily make a larger batch of these as suits your needs, just multiply everything up. To make in an 8in/20cm square tin, for example, use 1 ½ times the amounts, so with 3 eggs, ¾cup chocolate etc.