1) Preheat the oven to 300°F. For thin, crisp shortbread, grease two 8" square pans. For thicker shortbread, grease one 9" square pan.
2) To avoid over-creaming, we recommend that you stir the dough by hand. In a medium-sized bowl, stir together the butter, salt, and sugar until well blended.
3) Add the cocoa, vanilla, baking powder, and flour, and mix until you have a smooth, slightly stiff batter.
4) Press the dough into your prepared pan(s) and prick all over with a fork. This will keep the shortbread from developing bubbles under the crust.
5) Bake the shortbread in two pans for 35 to 40 minutes; the single 9" pan will take about 50 minutes. Remove from the oven and cool for 5 minutes.
6) Invert the pan(s) over a clean sheet of parchment and turn out the shortbread. While still warm, cut each square into 12 pieces. Cool completely before serving.
Yield: 12 to 24 pieces, depending on thickness.