Preheat oven to 350°F (180°C). Grease and line 13- x 9-inch (33 x 23 cm) baking pan with enough parchment paper to overhang edges; set aside.
Whisk together cocoa powder, flour, cornstarch, baking powder and salt; set aside.
In heatproof bowl set over saucepan of barely simmering water (do not allow bottom of bowl to touch simmering water), melt chopped chocolate and butter, stirring until smooth.
Stir in granulated sugar and vanilla sugar. Stir in eggs, one at a time, until fully incorporated after each addition. Stir in flour mixture; fold in pecans and white chocolate chips.
Transfer to prepared pan; smooth top. Bake for about 25 minutes or until only a few moist crumbs adhere to toothpick when inserted into center of brownie.
Mix together icing sugar, vanilla sugar and 4 tsp (20 mL) water to make thick paste; transfer to piping bag fitted with small round tip (or spoon into small resealable plastic bag, then snip bottom corner).
Cut brownie into 16 triangles; decorate with icing and sprinkles. Break candy canes into 16 (1 1/2 -inch/4 cm) lengths. Insert piece of candy cane into base of each tree to resemble tree trunks.