Preheat oven to 375 degrees. Line a jellyroll pan with parchment paper. Set aside. Place butter,splenda,vanilla and salt in a medium mixing bowl. If you have it mix with the paddle attachment of an electric mixer about 1 to 1 1/2 minutes
add cocoa powder and flour. Mix until just blended, Remove dough from bowl and form into a ball. Place the ball of dough on parchment-lined pan,. Roll the dough into a rectangle approximately 6 1/2 inches wide by 11 inches long and 1/4 inch thick. Pierce the suface of dough all over with a fork. This allows air to escape during baking preventing air pockets from forming,)
Bake in preheated oven 20 to 25 minutes rotating pan. After 10 minutes of baking, Remove shortbread from oven after 20 to 25 minutes and immediately cut into 24 fingers or rectangeles while still warm. If allowed to cool. shortbread will not slice well.