Preheat oven to 350°F (170°C).

Using a mixer with the paddle attachment, beat the margarine and butter together for about 30 seconds. Add brown sugar, granulated sugar, eggs, vanilla, baking soda, salt and 1 ¼ cups of the flour and beat until thoroughly combined (for about 1 minute), pausing once to scrape the sides of the bowl. Beat in the remaining 1 ¼ cups flour. Stir in pecans and white chocolate chunks.

Drop dough mounds of about 1 inch (2.5 cm) in diameter onto an ungreased baking tray. Bake at 350°F (170°C) for about 10 minutes, until edges are firm and starting to brown and the centers are no longer shiny. Cool on baking tray for 2 minutes then carefully loosen cookies from the tray and move them to wire racks to cool. Store in an airtight container at room temperature for up to three days or in the freezer for up to one month.