Preheat the oven to 350°F degrees. Line a 13x9 inch baking pan with foil or parchment paper, extending about 1 inch over the edge. Grease and set pan aside.

For the brownie layer, in a medium saucepan, heat 1/2 cup butter and unsweetened chocolate over low heat until melted and smooth. Remove from heat and set aside to cool slightly. Stir one cup of sugar into melted chocolate mixture. Add eggs one at a time, stirring after each addition until combined. Stir in vanilla.

In a small bow, stir together flour and baking soda and to chocolate mixture. Stir until just combined. Pour into prepared baking pan and bake in preheated oven for 15-17 minutes or until a toothpick inserted in the middle comes out clean. Cool completely in the pan on a wire rack. Place in the freezer while you make the caramel layer.

For the caramel layer, in a large microwave safe bowl, combine the caramels, evaporated milk and water. Microwave on high for 30 seconds. Stir and repeat until mixture is completely melted. Stir in crushed pretzels. Pour and spread caramel layer over cooled brownie layer, spreading to the edges. Place the pan in the freezer while preparing the nougat layer to cool completely.

For the nougat layer, in a medium saucepan, combine sugar, evaporated milk, and butter. Bring to a boil over medium high heat, stirring constantly. Once boiling reduce heat to medium and boil at a moderate steady rate without stirring for 10 minutes. While boiling, in a large bowl, combine the marshmallow fluff and peanut butter together. Whisk the hot sugar mixture into marshmallow mixture until well combined. Pour the nougat layer over caramel layer from the freezer. Place back in the freezer while preparing chocolate layer.

For the chocolate layer, in a small saucepan heat and stir semisweet chocolate pieces, butter and whipping cream over low heat until melted and smooth. Pour chocolate layer over nougat layer and spread to the edges. Cover and chill for at least 2 hours.

Using the edges of the foil or parchment paper, lift uncut brownies out of the pan. Cut and serve.

Store in the refrigerator for up to 3 days or in the freezer for up to two months.