Preheat oven to 325°F. Line a nine-inch (23-cm) square baking pan with parchment (or spray with oil and dust with flour).
In a large saucepan, melt chocolate and butter over low heat, stirring often.
Meanwhile, in a small bowl, combine flour with salt and baking powder.
Once chocolate has melted, remove from heat and stir in sugar until evenly mixed, then add vanilla, beet purée and eggs. When well mixed, fold in flour mixture, then scrape batter into the prepared pan. Bake for 35 minutes. Cool, then dust with icing sugar, if desired.
How to make beet puree
Boil two medium unpeeled beets until very tender. Drain, run under cold water to cool, then carefully slip off the skins. (Or wrap beets in foil and roast in preheated 375 degree F oven until tender. When cool enough to handle, slip off skins.) Coarsely chop beets, then whirl in food processor until you get a smooth puree. This can be refrigerated for five days, or frozen for up to three months (thaw before using).
Per brownie: 142 calories, 2 g protein, 9 g fat (6 g saturated fat), 15 g carbohydrates, 2 g fibre, 42 mg cholesterol, 56 mg sodium