For the crust, combine flour, powdered sugar and baking soda. Add the butter and mix until crumbly. Lightly press the crumb mixture into the bottom of a 9x13 pan. Bake for 15 minutes.
Meanwhile, for the filling, reserve 1 cup of the pecans for garnish; chop the remaining pecans. Combine the chopped pecans, brown sugar, eggs, butter, vanilla, and salt. Mix well. Pour filling over the warm crust, spreading to the edges of the pan.
Arrange the remaining pecan halves over the filling.
Bake 17-19 minutes or until filling is set in the center.
Remove from oven, cool completely.
Sprinkle with additional powdered sugar, if desired.
Cut into squares. Cut each square in half diagonally.