Cream butter with 1/2 cup powdered sugar and salt. Add flour and mix well. Pat it out in a greased 9x13 inch pan with your fingers.
Bake at 350 for 15 minutes. Remove from oven. Do not turn oven off.
Mix eggs with melted butter, and white sugar. Add baking ppowder, salt, and flour, and mix thoroughly.
Melt the chocolate chips in a double boiler or microwave. Be sure to stir and that the chips are completely melted.
Add the melted chocolate chips to your bowl and mix thoroughly.
Pour this mixture on the top of the pan you just baked and tip the pan so it covers all of the shortbread crust. Stick it back into the oven and bake it for another 25 mintues. Then remove it from the oven and sprinkle on the additon powered sugar.
Let it cool and cut into brownie-sized bars.
You can refrigerate these, but cut them before you do.