Preheat oven to 180C/350F. Grease and line an 8″ x 12″ baking tray {or something of a similar size}, leaving some baking paper overhanging to lift the brownie out of the tin once it's baked.
Whisk together all of the dry ingredients in a large bowl (chickpea flour through to cinnamon). Mix together wet ingredients (beetroot puree through to vanilla) in a food processor, or just whisk together in a bowl, until smooth. Whisk the wet mixture into the dry ingredients. Stir in the chopped chocolate if using then scrape into the prepared baking pan.
Bake in pre‐heated oven for approximately 25–30 minutes until a skewer inserted in the center comes out with some moist crumbs attached, but not too gooey (under‐baked is probably better than over‐baked though!). Remove from oven and place on a cooling rack. After the brownie has cooled for 15 minutes in the tin use your baking paper overhang to remove it, then place on the cooling rack until completely cool.
When completely cooled, frost and sprinkle with sea salt flakes. Slice into squares and serve. Brownies will keep for 3–4 days in the fridge in an airtight container but are best on the day of making.