Melt butter and chocolate squares in top of a double boiler over simmering water; set mixture aside to cool.
Combine eggs and 2 cups sugar; mix well. Combine flour and baking powder; mix well. Add to egg mixture, stirring well. Stir in chocolate mixture, pecans, and vanilla; mix well. Pour into a greased 13- x 9- x 2- inch baking pan. Bake at 325° for 35 minutes.
Combine powdered sugar and syrup; spread over warm brownies. Cool completely in pan before cutting into 2-inch squares.