On a baking sheet, toast the pecans in the middle of the oven, taking care to stir them from time to time. Set aside.
In a saucepan, over low heat, melt the chocolate and butter. Remove from the heat.
In a bowl, sift together the flour and baking powder. Set aside.
In another bowl, beat the eggs, sugar and vanilla with an electric mixer on high speed, until the mixture is light and fluffy, about 5 minutes. Stir the melted chocolate into the egg mixture while mixing on medium speed until the mixture is smooth. Stir in the dry ingredients on low speed.
With a wooden spoon, fold in the semisweet and white chocolate and pecans. For each cookie, place a generous 30 ml (2 tablespoons) of dough on a buttered baking sheet. Refrigerate the remaining dough for another batch. Bake in the middle of the oven for 12 to 14 minutes. Cool for 2 minutes before removing from the baking sheet. Cool completely on a rack.