To prepare the brownie batter -
Spray a non-stick 8" square pan with non-stick cooking spray; set aside. Combine the flour, baking soda and salt in a small bowl; stir and set aside. In a medium saucepan, combine the margerine, sugar and water. Cook on low, stirring constantly, just until the mixture comes to a boil. Remove from heat. Add the chocolate chips and stir until the chips are melted; set aside. In a mixing bowl, beat the eggs lightly. Stir in the vanilla, then add the melted chocolate mixture and stir until well blended. Add the flour mixture gradually, mixing well after each addition. Spoon about two-thirds of the batter into the prepared pan, spread it evenly over the bottom of the pan. Spoon the mint marble filling over top. Very gently spread it over the batter but do not it stir it in. Drop the remaining chocolate batter, by spoonfuls, on to the mint filling. Using a thin spatula of a table knife, gently swirl the batter and the filling together for a marble effect. do not over mix. Bake in a preheated oven at oven to 350° for 35 to 40 minutes or until the center is well set... a few moist crumbs will cling to a toothpick inserted in the center, but no wet batter. Place on a wire rack to cool completely in the pan. Cut into 16 squares.