Cream the butter.
Cream in the powdered sugar, the cocoa and the vanilla.
Now work in the flour.
Knead the dough on an unfloured board until smooth.
Spray the shortbread pan very lightly with a non-stick vegetable oil spray. 
Firmly press the dough into the shortbread pan.
Prick the entire surface with a fork, and bake the shortbread right in the pan at 325° for about 30 - 35 minutes.
Don't let the edges get too dark.
Let the shortbread cool in its pan for 
about 10 minutes before you loosen the edges with a knife and flip the pan over onto a wooden cutting board. If the 
shortbread does not come right out, tap one edge of the pan. 
Cut the shortbread into serving pieces while it is still warm.