Preheat the oven to 180° C (350 ° F). Line a small baking tray (ca. 20×30 cm) with buttered parchment paper.
Using an electric hand mixer or a table-top mixer whisk the butter and sugars together until creamy and fluffy. Add cacao, maple syrup and salt using a rubber spatula.
Add the eggs (one at a time) to the mixture. When all the egg has been incorporated, gently fold in the buckwheat flour and the chopped chocolate still using your rubber spatula.
Scrape the mixture into the baking tray and spread it out very gently. Bake for about 20-25 minutes until a skewer inserted in the middle comes out clean or just a little sticky. Let cool a bit before lifting the cake out of the tray. Then cool completely on a wire rack.
For the frosting
Bring cream, milk and maple syrup just to the boil and then pour it over the chopped chocolate. Let sit for a minute or two and then stir until smooth and well mixed. Add the salt and let cool.
When the cake is completely cooled, spread the frosting over the cake. Sprinkle with the chopped pistachios and put in the fridge to set for 30-60 minutes before cutting it.
Decorate with pansies (or other edible flowers) and enjoy! The brownies keep well in the freezer for up to three months.