Heat the oven to 350°F. In a 1-quart saucepan, heat 1 1/2 cups of the chocolate chips over low heat, stirring constantly, until melted. Cool to room temperature, about 15 minutes, but do not allow chocolate to become firm. Meanwhile, cut the white chocolate baking bars into 1/4- to 1/2-inch chunks; set aside.
In a large bowl, beat the butter, brown sugar and vanilla with an electric mixer on medium speed until light and fluffy. Beat in the eggs and melted chocolate until light and fluffy. With a wooden spoon, stir in the flour, baking soda and salt. Stir in the remaining 2 1/2 cups chocolate chips, the white chocolate chunks and pecan halves.
For each cookie, spoon dough into a 1/4-cup dry-ingredient measuring cup and level off with a knife. On an ungreased cookie sheet, drop the dough about 2 inches apart.
Bake 12 to 14 minutes or until set (centers will appear soft and moist). Cool on cookie sheet 2 minutes, then remove from cookie sheet to a cooling rack, using a turner. Cool cookie sheets between batches.