Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 11x17 inch baking pan.
In a large bowl, beat together 1 1/2 cups of margarine with sugar until light and fluffy; beat in the eggs, one at a time,fully incorporating each egg before adding the next. Mix in 1 cup of cocoa powder, the flour, and 1 1/2 teaspoons of salt. Stir in 1 tablespoon of vanilla extract and the chopped walnuts, and scoop the brownie batter into the prepared baking pan.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 30 minutes; set on a rack to cool.
To make the first frosting layer, beat 1/2 cup of margarine until fluffy, add the instant pudding powder and 3 tablespoons milk and mix until smooth. Gradually beat in 2 cups of confectioners' sugar. Spread the frosting on the cooled brownies and set them aside to let the frosting firm up.
For the second frosting layer, mix together 3 tablespoons of melted margarine with 1/2 cup of cocoa powder; stir in 1/3 cups milk and gradually beat in 3 1/2 cups of confectioners' sugar. Add more milk as necessary to make a smooth, spreadable frosting. Spread the second frosting on top of the first.