Preheat oven to 170 C / Gas 3. Grease and flour a 26cm tube cake tin. Mix together the flour and baking powder; set aside.
In a large bowl, cream together the butter, dark brown soft sugar and caster sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Stir in the chopped pecans. Pour batter into prepared tin.
Bake in the preheated oven for 60 to 75 minutes or until a skewer inserted into the centre of the cake comes out clean. Let cool in tin for 10 minutes, then turn out onto a wire rack and cool completely.