Preheat the oven to 350?F. Using 2 tablespoons of the butter, grease an 8-inch-square baking pan.
In a metal bowl set over a pot of simmering water, melt the remaining 1/2 cup butter with the white chocolate.
In a medium bowl whisk together the eggs and sugar. Add the flour, vanilla, and salt and mix well. Fold in the pecans and the melted white chocolate.
Spread the brownie mixture in the buttered pan and sprinkle the chopped semisweet chocolate over the top. Bake until firm, for about 25 minutes. Remove from the oven and allow to cool on a rack before cutting into squares.
Yield: about 16 brownies