To make the crust: In a medium-sized mixing bowl, whisk together the flours, salt, and sugar. Work in the cold butter until coarse crumbs form. Add the water and toss until the mixture is the texture of wet sand., Press the dough into the bottom and up the sides of a lightly greased standard (4 1/4" x 13 3/4") rectangular tart pan or a 9" round tart pan. Use a fork to prick the dough all over., Freeze the tart shell for 30 minutes (or longer), covered., Fifteen minutes prior to taking the tart out of the freezer, preheat the oven to 375°F., Bake the crust for 18 to 22 minutes, until golden brown. Set it aside to cool., To make the filling: Place the chocolate and butter in a heatproof bowl. Set aside., In a saucepan, whisk together the heavy cream and malted milk powder until well combined; don't worry about small lumps., Bring the cream mixture to a simmer, whisking to help dissolve any lumps, then pour it through a fine-mesh strainer over the chocolate and butter. Let sit for 1 minute, then stir until the mixture is smooth., Set the filling aside to cool for 10 to 15 minutes, then pour it into the baked crust., Refrigerate the tart for at least 4 hours, or overnight., Serve chilled — as is, or with a bit of whipped cream.