Position a rack in the center of the oven. Preheat oven to 400 degrees. Line a 13x18" sheet pan with parchment paper.
Lay the puff pastry in the pan on top of the parchment. Bake for 10-12 minutes, until it puffs and turns a slight golden brown. Set aside briefly to cool, switch oven to broil setting.
Tear puff pastry into pieces and lay 3/4ths of them in a 9x13 baking dish. Sprinkle the toppings in an even layer.
In a medium saucepan, combine the milk, granulated sugar, and vanilla and bring to a boil, stirring until sugar dissolves. Pour hot milk over pastry and nuts, then arrange remaining pastry on top, pushing them down into the mixture.
In a medium bowl, and using an electric hand mixer, beat the cream until it thickens and stiff peaks form, 5-7 minutes. Spread whipped cream on top and sprinkle brown sugar on top.
Place baking dish on the center rack of the oven and broil 3-6 minutes, watching carefully for the top to turn a golden brown.