Use The BIG food processor for this recipe. Grind the nuts first and place in a bowl off to the side.
Preheat the oven to 350°F. Prepare a 9x13 inch baking pan by greasing the bottom and sides with a little butter. Line with a wide strip of parchment paper, leaving two 'tails' at either side to help you lift the cooked brownie out of the pan.
Break up the chocolate into squares and put into a heatproof bowl along with the butter. Set over a pan of barely simmering water and allow to gently melt, stirring to mix together to combine.
Put the beets into a food processor and process to a puree, turning off occasionally to scrape down the sides if necessary. Add the melted chocolate and butter and the eggs and puree again until combined. Again, scrape the sides down, then add the sugar, ground almonds, cocoa, baking powder and chili powder (if using) and process until you have a smooth batter.
Pour into the prepared baking pan and bake for about 30 minutes until the top is set and the brownie is starting to pull a little away from the sides of the pan. A toothpick or knife inserted into the center should come out fairly clean but just a bit sticky- don't worry if it still seems a little soft as it will firm up on cooling. Remove from the oven, place on a cooling rack and allow to completely cool in the pan.
Using the parchment paper tails, ease the brownie from the pan and cut into squares. Dust the surface with a little confectioner's sugar.