Line bottom and sides of 13- x 9-inch (3 L) metal baking pan with parchment paper, leaving a 2-inch (5 cm) overhang at each end. 

Scrub sweet potato, trim off ends and pierce with small knife in several places. Microwave on High for 6 to 8 minutes or until tender, turning over halfway through. When cool enough to handle; remove skin and mash with fork until smooth. Measure out 1 cup (250 mL) and place in large bowl, set aside. 

In small microwavable bowl, combine chocolate and butter. Microwave on Medium for 1 to 2 minutes or until butter is melted. Stir until chocolate is completely melted; set aside to cool slightly. 

Add brown sugar and maple syrup to sweet potato and with wooden spoon, beat until smooth. Beat in eggs, one at a time. Stir in chocolate mixture and vanilla until smooth.  

In medium bowl, combine flour, cocoa, baking powder and salt; stir into chocolate mixture until dry ingredients are moistened. Spread evenly in prepared pan. Bake in 350°F (180°C) for 20 to 25 minutes or until toothpick inserted in centre comes out clean. Let cool completely in pan on wire rack. Using parchment overhang, lift brownie from pan. Remove parchment and place brownie on large platter. 

Icing: In medium microwaveable bowl, place chocolate. Microwave on Medium for 2 to 3 minutes or until chocolate is partially melted. Stir until chocolate is melted. Stir in yogurt and vanilla until smooth. Stir in icing sugar until smooth.  Add either more yogurt or icing sugar if needed for a spreadable consistency. Spread evenly over brownie; set aside to let icing firm up.