In a large bowl, combine the first five ingredients; stir in butter. Pat into a greased 11x7-in. baking pan. Bake at 350° for 5 minutes.
In another bowl, beat the sugar, butter and chocolate. Beat in egg and vanilla. Combine the flour, baking powder and salt; gradually add to the chocolate mixture alternately with milk, mixing well after each addition. Stir in pecans; spread over bottom layer.
Bake at 350° for 25 minutes. Cool.
For frosting, in a large microwave-safe bowl, melt chocolate and butter; stir until smooth. Stir in confectioners' sugar and vanilla (mixture will be very crumbly). Beat in enough water (about 2 tablespoons) to achieve desired spreading consistency. Immediately frost brownies and top with pecans halves.