Position a rack in the center of the oven and preheat to 160ºC .
Line a 33 cm by 24 cm baking pan with a double thickness of aluminum foil so that the foil extends 5 cm beyond the two short ends of the pan. Fold the overhang down along the sides of the pan. Butter the bottom of the foil-lined pan. In a medium bowl, stir together the flour, baking soda, baking powder and salt.
Place the semisweet chocolate in a large bowl. In a medium saucepan, combine the sugar, butter, corn syrup and water. Cook over medium heat, stirring constantly with a wooden spoon, until the butter melts, the sugar is dissolved and the mixture comes to a boil. Remove the pan from the heat and pour the hot syrup over the chocolate.
Let the mixture stand for 1 to 2 minutes, to melt the chocolate. Whisk until smooth. One at a time, whisk in the eggs, blending until smooth. Whisk in the vanilla and the flour mixture, mixing until the batter is smooth.
Using a rubber spatula, fold in 1 cup of the walnuts and 6 ounces of the Swiss chocolate chunks. Scrape the batter into the prepared pan and spread it evenly with a spatula.
Sprinkle the remaining half cup of walnuts and 3 ounces of chocolate chunks over the top of the batter.
Bake the brownies for 40 to 50 minutes, or until a cake tester or toothpick inserted into the center of the brownie comes out with a few moist crumbs clinging to it.
Cool the brownies in the pan set on a wire rack. When the brownies are completely cool, cover the pan of brownies with plastic wrap and let the brownies set at room temperature for at least 6 hours or overnight.
Invert the brownies onto a large plate or cutting board.
Remove the pan and carefully peel off the foil.
Invert again onto a smooth cutting surface and cut into 24 bars.
Store the brownies at room temperature in an airtight container for up to five days.