Preheat oven to 375°F. Line a baking sheet with parchment.
In the bowl of a stand mixer, combine butter, Splenda Sweetener, sugar, vanilla extract, and salt. Mix until light and creamy, about 2 minutes. Add cocoa powder and flour; mix until blended.
Gather the dough into a ball and place on baking sheet. Roll dough into a rectangle about 6 ½” x 11” and ¼” thick. Use a fork to prick the surface of the dough all over.
Bake for 20–25 minutes, rotating the pan after 10 minutes.
Remove shortbread from oven and immediately cut into 24 fingers or rectangles. (Shortbread slices more easily when warm.)