Cook first 3 ingredients in a large saucepan over high heat, stirring constantly, until sugar melts. Add 2 cups chocolate morsels and vanilla extract, stirring until mixture is smooth. Let cool 15 minutes.
Add flour, baking soda, and salt to cooled chocolate mixture, stirring until blended; stir in eggs and chopped pecans until blended. Spread brownie batter into a greased and floured 13- x 9-inch pan.
Bake at 325° for 30 minutes. Sprinkle remaining 2 cups chocolate morsels evenly over warm brownies, and let stand 5 minutes to soften. Spread over top. Cool on a wire rack.