White chocolate ganache, part 1: In a double boiler or heatproof bowl set over (but not touching) barely simmering water, melt the chocolate, stirring occasionally with a rubber spatula. Whisk in cream in three stages until homogeneous after each addition. Whisk in vanilla, then add salt. Taste for balance and add a pinch more salt as needed. Strain through a fine-mesh strainer. Refrigerate overnight.
Brownie pie: Preheat oven to 350 F. In a double boiler or heatproof bowl set over (but not touching) barely simmering water, melt the chocolate and butter, stirring occasionally with rubber spatula. Meanwhile, in a separate bowl, whisk together both sugars. Pour the warm chocolate mixture into the bowl with the sugars. Whisk to combine. Put the eggs and vanilla in another bowl. Whisk this mixture one egg at a time into the chocolate. Finish by scraping the sides and bottom of the bowl with a rubber spatula to bring the batter together.
Add the dry ingredients: flour, cocoa, baking powder, and salt. Slowly whisk to combine; then switch to the rubber spatula to bring the batter together. Spray a 9-inch pie plate with nonstick spray, then fill halfway with batter. Bake 20 minutes, until brownies are set—firm to the touch but still slightly soft in the middle. (You’ll have enough batter leftover to pour it into an 8-inch baking dish and bake at 350 F for 20 minutes.)
Chocolate candy cane ganache: While brownie pie is cooling, make the ganache. In a small, heavy-bottomed saucepan, bring the cream to a boil over medium-high heat, stirring occasionally. Place the chocolate in a bowl and pour the boiling cream over it. Let it rest to melt the chocolate, about 1 minute. Whisk to bring together, then whisk in the vanilla and salt. Pour over the cooled brownies and spread evenly to ½-inch from the edge. Scatter the chopped candy canes on top. Refrigerate to set, 1 hour.
White chocolate ganache, part 2: In a stand mixer fitted with a whisk attachment, whip the white chocolate ganache mixture over medium speed, 2 minutes. When the ganache thickens, turn the speed to high and continue whipping until it looks like whipped cream, another 2 minutes.
Assembly: Place whipped ganache into a pastry bag fitted with a star tip. Pipe stars to cover the pie completely. Note: Piping is an optional step. Ganache can also be spread over the top in an even layer. Serve.