Preheat the oven to 300°F (150°C).

Lightly grease two 9-inch round cake pans.

In a medium bowl, cream together the unsalted butter, salt, sugar, and vanilla extract until light and fluffy.

Beat in the dutch process cocoa, baking powder, and flour until just combined.

Divide the dough in half.

Press each half into the prepared pans, smoothing the surface with your fingers.

Prick the dough with a fork in an attractive pattern.

Bake for 35-40 minutes, or until the edges appear done.

Remove from the oven.

Loosen the edges with a heat-resistant plastic knife or table knife.

Wait 5 minutes.

Carefully turn the shortbread out onto a clean work surface in one piece.

Using a pizza wheel, baker's bench knife, or sharp knife, cut each round into 12 wedges while still warm.

Transfer the wedges to a rack to cool completely.