Filling and Assembly
Preheat oven to 350 degrees F.
Toss walnut pieces with ¼ cup maple syrup and spread on a parchment-lined baking sheet. Toast for 12 minutes, until browned. Cool and chop finely by hand and set aside.
Preheat oven to 275 degrees F. and trace three 9-inch circles on a sheet of parchment paper. Place paper upside on baking sheet.
Whip egg whites until foamy and, while still whipping, pour in sugar and whip whites until they hold a stiff peak. Whip in brown sugar quickly. Spread meringue over the three circles, spreading to just a ½ -inch inside the edge of each circle (the meringue will expand as it bakes). Bake for about an hour, until crisp (a little bit of browning is okay). Allow to cool.
Whip the cream to medium peaks and fold in 2/3 cup maple syrup and all but 3 Tbsp of chopped maple walnuts.
Grease and line a 9-inch springform pan with parchment paper.
Place one meringue disc at the bottom of the pan and spread with a third of the cream. Top with second meringue disc and top with cream. Repeat with third disc and sprinkle top layer of cream with reserved 3 Tbsp chopped maple walnuts. Freeze overnight.
To serve, place torte in the fridge an hour before serving, for easier slicing and to soften the meringue layers.