Position a rack in the center of the oven and preheat to 325 degrees F. Lightly
butter the bottom and sides of a 12-inch diameter pizza pan.
Melt unsweetened chocolate, then cool until the chocolate is tepid.
Chop the bittersweet and milk chocolates into 1/2-inch squares and set aside.
In a small bowl, using a wire whisk, stir together the flour and salt.
In a large bowl, using a hand-held electric mixer set at medium speed, beat the
butter and brown sugar for 1 to 2 minutes, until the mixture is light in texture
and beige in color.
One at a time, beat in the eggs, mixing well after each addition.
Beat in the melted chocolate and vanilla extract until blended.
At low speed, beat in the flour mixture just until combined.
Using a rubber spatula, fold in half of the chopped bittersweet and milk chocolate
squares and half of the pecans. Scrape the batter into the prepared pan. Evenly
smooth the surface with a spatula.
Sprinkle the surface with the remaining chopped chocolate pieces and pecans.
Bake for 25 to 30 minutes, or until a wooden pick inserted
2 inches away from the center comes out clean.
Transfer the pan to a wire rack to cool.
Melt the white chocolate. Fill a small paper cone with the melted white chocolate
and cut a 1/3-inch opening at the tip. Drizzle the white chocolate over the surface
of the pan. Cut into wedges to serve.