Steam or boil beets until very tender, about 30 minutes. Place them in a food processor and purée; set aside to cool. Preheat oven to 350°F. Lightly grease an 8-in. square baking pan. Place chocolate and butter in a metal bowl over a pot of simmering water, and stir until melted (or place in a microwave-safe dish and microwave on high in 25-second increments, stirring between each interval, until melted).
Remove from heat, transfer to a large bowl and mix in eggs, one at a time.
Gently stir in beet purée, flour, sugar, yogurt, cocoa powder, cinnamon, vanilla, baking powder and salt. Fold in nuts if using. Pour mixture into pan and spread evenly. Bake for 20 minutes or until a toothpick inserted into centre comes out clean. Let cool before slicing.
Per brownie: 151 calories, 4 g protein, 9 g fat (6 g saturated fat), 18 g carbohydrates, 3 g fibre, 10 g sugars, 34 mg cholesterol, 75 mg sodium