Position a rack in the center of the oven and heat the oven to 325? F. Line the bottom and sides of a 9x13-inch straight-sided metal baking pan with heavy-duty aluminum foil, leaving about a 2-inch overhang on the short sides. Lightly butter the foil. Put the butter in a large (4-quart) saucepan over medium-low heat and stir occasionally until melted, about 2 minutes. Off the heat, whisk in the cocoa powder until smooth, 1 minute. Add the sugar and salt, and whisk until well blended. Use your fingertip to check the temperature of the batter -- it should be warm, not hot. If it's hot, set the pan aside for a minute or two before continuing. Whisk in the Kahlua and dark rum and then the eggs, two and then three at a time, until just blended. Sprinkle the flour over the batter and stir with a rubber spatula until just blended. Stir in the white chocolate chips and semisweet chocolate chips. Scrape the batter into the prepared pan and spread evenly. Drop the Nutella by tablespoons evenly o