Preheat oven to 180 degrees C. Grease a 20x20cm square cake tin.
In a medium bowl cream together the butter, brown sugar and white sugar. Beat in the eggs and vanilla. Stir in the flour and pecans until just blended. Spread the mixture evenly into the prepared cake tin.
Bake for 35 to 40 minutes in the preheated oven or until edges begin to pull away from the sides of the tin.